The Different Forms of Flowers on Plants of the Same Species
by
Charles Darwin
Part 4 out of 6
The pollen of four other plants, in which none of the anthers were contabescent,
was examined; in one a moderate number of grains were minute and shrivelled, but
in the other three they appeared perfectly sound. With respect to the power of
producing seed, five plants (Numbers 21 to 25) were observed: one yielded
scarcely more than half the normal number; a second was slightly infertile; but
the three others actually produced a larger average number of seeds, with a
higher maximum, than the standard. In my concluding remarks I shall recur to
this fact, which at first appears inexplicable.
[PLANT 21.
This short-styled plant, freely and legitimately fertilised during 1865 by
illegitimate plants, descended from self-fertilised long-, mid- and short-styled
parents, yielded an average from ten capsules of 43 seeds, with a maximum of 63
and a minimum of 26: hence this plant, which was the one with all its longer and
many of its shorter stamens contabescent, produced only 52 per cent of the
proper number of seeds.
PLANT 22.
This short-styled plant produced perfectly sound pollen, as viewed under the
microscope. During 1866 it was freely and legitimately fertilised by other
illegitimate plants belonging to the present and the following class, both of
which include many highly fertile plants. Under these circumstances it yielded
from eight capsules an average of 100.5 seeds, with a maximum of 123 and a
minimum of 86; so that it produced 121 per cent of seeds in comparison with the
normal standard. During 1864 it was allowed to be freely and legitimately
fertilised by legitimate and illegitimate plants, and yielded an average, from
eight capsules, of 104.2 seeds, with a maximum of 125 and a minimum of 90;
consequently it exceeded the normal standard, producing 125 per cent of seeds.
In this case, as in some previous cases, pollen from legitimate plants added in
a small degree to the fertility of the plant; and the fertility would, perhaps,
have been still greater had not the summer of 1864 been very hot and certainly
unfavourable to some of the plants of Lythrum.
PLANT 23.
This short-styled plant produced perfectly sound pollen. During 1866 it was
freely and legitimately fertilised by the other illegitimate plants specified
under the last experiment, and eight capsules yielded an average of 113.5 seeds,
with a maximum of 123 and a minimum of 93. Hence this plant exceeded the normal
standard, producing no less than 136 per cent of seeds.
PLANT 24.
This long-styled plant produced pollen which seemed under the microscope sound;
but some of the grains did not swell when placed in water. During 1864 it was
legitimately fertilised by legitimate and illegitimate plants in the same manner
as Plant 22, but yielded an average, from ten capsules, of only 55 seeds, with a
maximum of 88 and a minimum of 24, thus attaining 59 per cent of the normal
fertility. This low degree of fertility, I presume, was owing to the
unfavourable season; for during 1866, when legitimately fertilised by
illegitimate plants in the manner described under Number 22, it yielded an
average, from eight capsules, of 82 seeds, with a maximum of 120 and a minimum
of 67, thus producing 88 per cent of the normal number of seeds.
PLANT 25.
The pollen of this long-styled plant contained a moderate number of poor and
shrivelled grains; and this is a surprising circumstance, as it yielded an
extraordinary number of seeds. During 1866 it was freely and legitimately
fertilised by illegitimate plants, as described under Number 22, and yielded an
average, from eight capsules, of 122.5 seeds, with a maximum of 149 and a
minimum of 84. Hence this plant exceeded the normal standard, producing no less
than 131 per cent of seeds.]
CLASS 6. ILLEGITIMATE PLANTS RAISED FROM MID-STYLED PARENTS FERTILISED WITH
POLLEN FROM THE SHORTEST STAMENS OF THE LONG-STYLED FORM.
I raised from this union twenty-five plants, which proved to be seventeen long-
styled and eight mid-styled, but not one short-styled. None of these plants were
in the least dwarfed. I examined, during the highly favourable season of 1866,
the pollen of four plants: in one mid-styled plant, some of the anthers of the
longest stamens were contabescent, but the pollen-grains in the other anthers
were mostly sound, as they were in all the anthers of the shortest stamens; in
two other mid-styled and in one long-styled plant many of the pollen-grains were
small and shrivelled; and in the latter plant as many as a fifth or sixth part
appeared to be in this state. I counted the seeds in five plants (Numbers 26 to
30), of which two were moderately sterile and three fully fertile.
[PLANT 26.
This mid-styled plant was freely and legitimately fertilised, during the rather
unfavourable year 1864, by numerous surrounding legitimate and illegitimate
plants. It yielded an average, from ten capsules, of 83.5 seeds, with a maximum
of 110 and a minimum of 64, thus attaining 64 per cent of the normal fertility.
During the highly favourable year 1866, it was freely and legitimately
fertilised by illegitimate plants belonging to the present Class and to Class 5,
and yielded an average, from eight capsules, of 86 seeds, with a maximum of 109
and a minimum of 61, and thus attained 66 per cent of the normal fertility. This
was the plant with some of the anthers of the longest stamens contabescent as
above mentioned.
PLANT 27.
This mid-styled plant, fertilised during 1864 in the same manner as the last,
yielded an average, from ten capsules, of 99.4 seeds, with a maximum of 122 and
a minimum of 53, thus attaining to 76 per cent of the normal fertility. If the
season had been more favourable, its fertility would probably have been somewhat
greater, but, judging from the last experiment, only in a slight degree.
PLANT 28.
This mid-styled plant, when legitimately fertilised during the favourable season
of 1866, in the manner described under Number 26, yielded an average, from eight
capsules, of 89 seeds, with a maximum of 119 and a minimum of 69, thus producing
68 per cent of the full number of seeds. In the pollen of both sets of anthers,
nearly as many grains were small and shrivelled as sound.
PLANT 29.
This long-styled plant was legitimately fertilised during the unfavourable
season of 1864, in the manner described under Number 26, and yielded an average,
from ten capsules, of 84.6 seeds, with a maximum of 132 and a minimum of 47,
thus attaining to 91 per cent of the normal fertility. During the highly
favourable season of 1866, when fertilised in the manner described under Number
26, it yielded an average, from nine capsules (one poor capsule having been
excluded), of 100 seeds, with a maximum of 121 and a minimum of 77. This plant
thus exceeded the normal standard, and produced 107 per cent of seeds. In both
sets of anthers there were a good many bad and shrivelled pollen-grains, but not
so many as in the last-described plant.
Plant 30.
This long-styled plant was legitimately fertilised during 1866 in the manner
described under Number 26, and yielded an average, from eight capsules, of 94
seeds, with a maximum of 106 and a minimum of 66; so that it exceeded the normal
standard, yielding 101 per cent of seeds.
Plant 31.
Some flowers on this long-styled plant were artificially and legitimately
fertilised by one of its brother illegitimate mid-styled plants; and five
capsules yielded an average of 90.6 seeds, with a maximum of 97 and a minimum of
79. Hence, as far as can be judged from so few capsules, this plant attained,
under these favourable circumstances, 98 per cent of the normal standard.]
CLASS 7. ILLEGITIMATE PLANTS RAISED FROM MID-STYLED PARENTS FERTILISED WITH
POLLEN FROM THE LONGEST STAMENS OF THE SHORT-STYLED FORM.
It was shown in the last chapter that the union from which these illegitimate
plants were raised is far more fertile than any other illegitimate union; for
the mid-styled parent, when thus fertilised, yielded an average (all very poor
capsules being excluded) of 102.8 seeds, with a maximum of 130; and the
seedlings in the present class likewise have their fertility not at all
lessened. Forty plants were raised; and these attained their full height and
were covered with seed-capsules. Nor did I observe any contabescent anthers. It
deserves, also, particular notice that these plants, differently from what
occurred in any of the previous classes, consisted of all three forms, namely,
eighteen short-styled, fourteen long-styled, and eight mid-styled plants. As
these plants were so fertile, I counted the seeds only in the two following
cases.
[PLANT 32.
This mid-styled plant was freely and legitimately fertilised during the
unfavourable year of 1864, by numerous surrounding legitimate and illegitimate
plants. Eight capsules yielded an average of 127.2 seeds, with a maximum of 144
and a minimum of 96; so that this plant attained 98 per cent of the normal
standard.
PLANT 33.
This short-styled plant was fertilised in the same manner and at the same time
with the last; and ten capsules yielded an average of 113.9, with a maximum of
137 and a minimum of 90. Hence this plant produced no less than 137 per cent of
seeds in comparison with the normal standard.]
CONCLUDING REMARKS ON THE ILLEGITIMATE OFFSPRING OF THE THREE FORMS OF Lythrum
salicaria.
From the three forms occurring in approximately equal numbers in a state of
nature, and from the results of sowing seed naturally produced, there is reason
to believe that each form, when legitimately fertilised, reproduces all three
forms in about equal numbers. Now, we have seen (and the fact is a very singular
one) that the fifty-six plants produced from the long-styled form,
illegitimately fertilised with pollen from the same form (Class 1 and 2), were
all long-styled. The short-styled form, when self-fertilised (Class 3), produced
eight short-styled and one long-styled plant; and the mid-styled form, similarly
treated (Class 4), produced three mid-styled and one long-styled offspring; so
that these two forms, when illegitimately fertilised with pollen from the same
form, evince a strong, but not exclusive, tendency to reproduce the parent-form.
When the short-styled form was illegitimately fertilised by the long-styled form
(Class 5), and again when the mid-styled was illegitimately fertilised by the
long-styled (Class 6), in each case the two parent-forms alone were reproduced.
As thirty-seven plants were raised from these two unions, we may, with much
confidence, believe that it is the rule that plants thus derived usually consist
of both parent-forms, but not of the third form. When, however, the mid-styled
form was illegitimately fertilised by the longest stamens of the short-styled
(Class 7), the same rule did not hold good; for the seedlings consisted of all
three forms. The illegitimate union from which these latter seedlings were
raised is, as previously stated, singularly fertile, and the seedlings
themselves exhibited no signs of sterility and grew to their full height. From
the consideration of these several facts, and from analogous ones to be given
under Oxalis, it seems probable that in a state of nature the pistil of each
form usually receives, through the agency of insects, pollen from the stamens of
corresponding height from both the other forms. But the case last given shows
that the application of two kinds of pollen is not indispensable for the
production of all three forms. Hildebrand has suggested that the cause of all
three forms being regularly and naturally reproduced, may be that some of the
flowers are fertilised with one kind of pollen, and others on the same plant
with the other kind of pollen. Finally, of the three forms, the long-styled
evinces somewhat the strongest tendency to reappear amongst the offspring,
whether both, or one, or neither of the parents are long-styled.
[TABLE 5.30. Tabulated results of the fertility of the foregoing illegitimate
plants, when legitimately fertilised, generally by illegitimate plants, as
described under each experiment. Plants 11, 12 and 13 are excluded, as they were
illegitimately fertilised.
NORMAL STANDARD OF FERTILITY OF THE THREE FORMS, WHEN LEGITIMATELY AND NATURALLY
FERTILISED.
Column 1: Form.
Column 2: Average number of seeds per capsule.
Column 3: Maximum number in any one capsule.
Column 4: Minimum number in any one capsule.
Long-styled : 93 : 159 : No record was kept as all very poor capsules were
rejected.
Mid-styled : 130 : 151 : No record was kept as all very poor capsules were
rejected.
Short-styled : 83.5 : 112 : No record was kept as all very poor capsules were
rejected.
TABLE 5.30. Continued.
CLASS 1 AND CLASS 2.--ILLEGITIMATE PLANTS RAISED FROM LONG-STYLED PARENTS
FERTILISED WITH POLLEN FROM OWN-FORM MID-LENGTH OR SHORTEST STAMENS.
Column 1: Number (name) of plant.
Column 2: Form.
Column 3: Average number of seeds per capsule.
Column 4: Maximum number of seeds in any one capsule.
Column 5: Minimum number of seeds in any one capsule.
Column 6: Average number of seeds, expressed as the percentage of the normal
standard.
1 : Long-styled : 0 : 0 : 0 : 0.
2 : Long-styled : 4.5 : ? : 0 : 5.
3 : Long-styled : 4.5 : ? : 0 : 5.
4 : Long-styled : 4.5 : ? : 0 : 5.
5 : Long-styled : 0 or 1 : 2 : 0 : 0 or 1.
6 : Long-styled : 0 : 0 : 0 : 0.
7 : Long-styled : 36.1 : 47 : 22 : 39.
8 : Long-styled : 41.1 : 73 : 11 : 44.
9 : Long-styled : 57.1 : 86 : 23 : 61.
10 : Long-styled : 44.2 : 69 : 25 : 47.
CLASS 3. ILLEGITIMATE PLANTS RAISED FROM SHORT-STYLED PARENTS FERTILISED WITH
POLLEN FROM OWN-FORM SHORTEST STAMENS.
14 : Short-styled : 28.3 : 51 : 11 : 33.
15 : Short-styled : 32.6 : 49 : 20 : 38.
16 : Short-styled : 77.8 : 97 : 60 : 94.
17 : Long-styled : 76.3 : 88 : 57 : 82.
CLASS 4. ILLEGITIMATE PLANTS RAISED FROM MID-STYLED PARENTS FERTILISED WITH
POLLEN FROM OWN-FORM LONGEST STAMENS.
18 : Mid-styled : 102.6 : 131 : 63 : 80.
19 : Mid-styled : 73.4 : 87 : 64 : 56.
20 : Long-styled : 69.6 : 83 : 52 : 75.
CLASS 5. ILLEGITIMATE PLANTS RAISED FROM SHORT-STYLED PARENTS FERTILISED WITH
POLLEN FROM THE MID-LENGTH STAMENS OF THE LONG-STYLED FORM.
21 : Short-styled : 43.0 : 63 : 26 : 52.
22 : Short-styled : 100.5 : 123 : 86 : 121.
23 : Short-styled : 113.5 : 123 : 93 : 136.
24 : Long-styled : 82.0 : 120 : 67 : 88.
25 : Long-styled : 122.5 : 149 : 84 : 131.
CLASS 6. ILLEGITIMATE PLANTS RAISED FROM MID-STYLED PARENTS FERTILISED WITH
POLLEN FROM THE SHORTEST STAMENS OF THE LONG-STYLED FORM.
26 : Mid-styled : 86.0 : 109 : 61 : 66.
27 : Mid-styled : 99.4 : 122 : 53 : 76.
28 : Mid-styled : 89.0 : 119 : 69 : 68.
29 : Long-styled : 100.0 : 121 : 77 : 107.
30 : Long-styled : 94.0 : 106 : 66 : 101.
31 : Long-styled : 90.6 : 97 : 79 : 98.
CLASS 7. ILLEGITIMATE PLANTS RAISED FROM MID-STYLED PARENTS FERTILISED WITH
POLLEN FROM THE LONGEST STAMENS OF THE SHORT-STYLED FORM.
32 : Mid-styled : 127.2 : 144 : 96 : 98.
33 : Short-styled : 113.9 : 137 : 90 : 137.
The lessened fertility of most of these illegitimate plants is in many respects
a highly remarkable phenomenon. Thirty-three plants in the seven classes were
subjected to various trials, and the seeds carefully counted. Some of them were
artificially fertilised, but the far greater number were freely fertilised (and
this is the better and natural plan) through the agency of insects, by other
illegitimate plants. In the right hand, or percentage column, in Table 5.30, a
wide difference in fertility between the plants in the first four and the last
three classes may be perceived. In the first four classes the plants are
descended from the three forms illegitimately fertilised with pollen taken from
the same form, but only rarely from the same plant. It is necessary to observe
this latter circumstance; for, as I have elsewhere shown, most plants, when
fertilised with their own pollen, or that from the same plant, are in some
degree sterile, and the seedlings raised from such unions are likewise in some
degree sterile, dwarfed, and feeble. (5/3. 'The Effects of Cross and Self-
fertilisation in the Vegetable Kingdom' 1876.) None of the nineteen illegitimate
plants in the first four classes were completely fertile; one, however, was
nearly so, yielding 96 per cent of the proper number of seeds. From this high
degree of fertility we have many descending gradations, till we reach an
absolute zero, when the plants, though bearing many flowers, did not produce,
during successive years, a single seed or even seed-capsule. Some of the most
sterile plants did not even yield a single seed when legitimately fertilised
with pollen from legitimate plants. There is good reason to believe that the
first seven plants in Class 1 and 2 were the offspring of a long-styled plant
fertilised with pollen from its own-form shortest stamens, and these plants were
the most sterile of all. The remaining plants in Class 1 and 2 were almost
certainly the product of pollen from the mid-length stamens, and although very
sterile, they were less so than the first set. None of the plants in the first
four classes attained their full and proper stature; the first seven, which were
the most sterile of all (as already stated), were by far the most dwarfed,
several of them never reaching to half their proper height. These same plants
did not flower at so early an age, or at so early a period in the season, as
they ought to have done. The anthers in many of their flowers, and in the
flowers of some other plants in the first six classes, were either contabescent
or included numerous small and shrivelled pollen-grains. As the suspicion at one
time occurred to me that the lessened fertility of the illegitimate plants might
be due to the pollen alone having been affected, I may remark that this
certainly was not the case; for several of them, when fertilised by sound pollen
from legitimate plants, did not yield the full complement of seeds; hence it is
certain that both the female and male reproductive organs were affected. In each
of the seven classes, the plants, though descended from the same parents, sown
at the same time and in the same soil, differed much in their average degree of
fertility.
Turning now to the fifth, sixth, and seventh classes, and looking to the right
hand column of Table 5.30, we find nearly as many plants with a percentage of
seeds above the normal standard as beneath it. As with most plants the number of
seeds produced varies much, it might be thought that the present case was one
merely of variability. But this view must be rejected, as far as the less
fertile plants in these three classes are concerned: first, because none of the
plants in Class 5 attained their proper height, which shows that they were in
some manner affected; and, secondly, because many of the plants in Classes 5 and
6 produced anthers which were either contabescent or included small and
shrivelled pollen-grains. And as in these cases the male organs were manifestly
deteriorated, it is by far the most probable conclusion that the female organs
were in some cases likewise affected, and that this was the cause of the reduced
number of seeds.
With respect to the six plants in these three classes which yielded a very high
percentage of seeds, the thought naturally arises that the normal standard of
fertility for the long-styled and short-styled forms (with which alone we are
here concerned) may have been fixed too low, and that the six legitimate plants
are merely fully fertile. The standard for the long-styled form was deduced by
counting the seeds in twenty-three capsules, and for the short-styled form from
twenty-five capsules. I do not pretend that this is a sufficient number of
capsules for absolute accuracy; but my experience has led me to believe that a
very fair result may thus be gained. As, however, the maximum number observed in
the twenty-five capsules of the short-styled form was low, the standard in this
case may possibly be not quite high enough. But it should be observed, in the
case of the illegitimate plants, that in order to avoid over-estimating their
infertility, ten very fine capsules were always selected; and the years 1865 and
1866, during which the plants in the three latter classes were experimented on,
were highly favourable for seed-production. Now, if this plan of selecting very
fine capsules during favourable seasons had been followed for obtaining the
normal standards, instead of taking, during various seasons, the first capsules
which came to hand, the standards would undoubtedly have been considerably
higher; and thus the fact of the six foregoing plants appearing to yield an
unnaturally high percentage of seeds may, perhaps, be explained. On this view,
these plants are, in fact, merely fully fertile, and not fertile to an abnormal
degree. Nevertheless, as characters of all kinds are liable to variation,
especially with organisms unnaturally treated, and as in the four first and more
sterile classes, the plants derived from the same parents and treated in the
same manner, certainly did vary much in sterility, it is possible that certain
plants in the latter and more fertile classes may have varied so as to have
acquired an abnormal degree of fertility. But it should be noticed that, if my
standards err in being too low, the sterility of all the many sterile plants in
the several classes will have to be estimated by so much the higher. Finally, we
see that the illegitimate plants in the four first classes are all more or less
sterile, some being absolutely barren, with one alone almost completely fertile;
in the three latter classes, some of the plants are moderately sterile, whilst
others are fully fertile, or possibly fertile in excess.
The last point which need here be noticed is that, as far as the means of
comparison serve, some degree of relationship generally exists between the
infertility of the illegitimate union of the several parent-forms and that of
their illegitimate offspring. Thus the two illegitimate unions, from which the
plants in Classes 6 and 7 were derived, yielded a fair amount of seed, and only
a few of these plants are in any degree sterile. On the other hand, the
illegitimate unions between plants of the same form always yield very few seeds,
and their seedlings are very sterile. Long-styled parent-plants when fertilised
with pollen from their own-form shortest stamens, appear to be rather more
sterile than when fertilised with their own-form mid-length stamens; and the
seedlings from the former union were much more sterile than those from the
latter union. In opposition to this relationship, short-styled plants
illegitimately fertilised with pollen from the mid-length stamens of the long-
styled form (Class 5) are very sterile; whereas some of the offspring raised
from this union were far from being highly sterile. It may be added that there
is a tolerably close parallelism in all the classes between the degree of
sterility of the plants and their dwarfed stature. As previously stated, an
illegitimate plant fertilised with pollen from a legitimate plant has its
fertility slightly increased. The importance of the several foregoing
conclusions will be apparent at the close of this chapter, when the illegitimate
unions between the forms of the same species and their illegitimate offspring,
are compared with the hybrid unions of distinct species and their hybrid
offspring.
OXALIS.
No one has compared the legitimate and illegitimate offspring of any trimorphic
species in this genus. Hildebrand sowed illegitimately fertilised seeds of
Oxalis Valdiviana, but they did not germinate (5/4. 'Botanische Zeitung' 1871
page 433 footnote.); and this fact, as he remarks, supports my view that an
illegitimate union resembles a hybrid one between two distinct species, for the
seeds in this latter case are often incapable of germination.
[The following observations relate to the nature of the forms which appear among
the legitimate seedlings of Oxalis Valdiviana. Hildebrand raised, as described
in the paper just referred to, 211 seedlings from all six legitimate unions, and
the three forms appeared among the offspring from each union. For instance,
long-styled plants were legitimately fertilised with pollen from the longest
stamens of the mid-styled form, and the seedlings consisted of 15 long-styled,
18 mid-styled, and 6 short-styled. We here see that a few short-styled plants
were produced, though neither parent was short-styled; and so it was with the
other legitimate unions. Out of the above 211 seedlings, 173 belonged to the
same two forms as their parents, and only 38 belonged to the third form distinct
from either parent. In the case of O. Regnelli, the result, as observed by
Hildebrand, was nearly the same, but more striking: all the offspring from four
of the legitimate unions consisted of the two parent-forms, whilst amongst the
seedlings from the other two legitimate unions the third form appeared. Thus, of
the 43 seedlings from the six legitimate unions, 35 belonged to the same two
forms as their parents, and only 8 to the third form. Fritz Muller also raised
in Brazil seedlings from long-styled plants of O. Regnelli legitimately
fertilised with pollen from the longest stamens of the mid-styled form, and all
these belonged to the two parent-forms. (5/5. 'Jenaische Zeitschrift' etc. Band
6 1871 page 75.) Lastly, seedlings were raised by me from long-styled plants of
O. speciosa legitimately fertilised by the short-styled form, and from the
latter reciprocally fertilised by the long-styled; and these consisted of 33
long-styled and 26 short-styled plants, with not one mid-styled form. There can,
therefore, be no doubt that the legitimate offspring from any two forms of
Oxalis tend to belong to the same two forms as their parents; but that a few
seedlings belonging to the third form occasionally make their appearance; and
this latter fact, as Hildebrand remarks, may be attributed to atavism, as some
of their progenitors will almost certainly have belonged to the third form.
When, however, any one form of Oxalis is fertilised illegitimately with pollen
from the same form, the seedlings appear to belong invariably to this form. Thus
Hildebrand states that long-styled plants of O. rosea growing by themselves have
been propagated in Germany year after year by seed, and have always produced
long-styled plants. (5/6. 'Ueber den Trimorphismus in der Gattung Oxalis:
Monatsberichte der Akad. der Wissen. zu Berlin' 21 June 1866 page 373 and
'Botanische Zeitung' 1871 page 435.) Again, 17 seedlings were raised from mid-
styled plants of O. hedysaroides growing by themselves, and these were all mid-
styled. So that the forms of Oxalis, when illegitimately fertilised with their
own pollen, behave like the long-styled form of Lythrum salicaria, which when
thus fertilised always produced with me long-styled offspring.]
PRIMULA.
Primula Sinensis.
I raised during February 1862, from some long-styled plants illegitimately
fertilised with pollen from the same form, twenty-seven seedlings. These were
all long-styled. They proved fully fertile or even fertile in excess; for ten
flowers, fertilised with pollen from other plants of the same lot, yielded nine
capsules, containing on an average 39.75 seeds, with a maximum in one capsule of
66 seeds. Four other flowers legitimately crossed with pollen from a legitimate
plant, and four flowers on the latter crossed with pollen from the illegitimate
seedlings, yielded seven capsules with an average of 53 seeds, with a maximum of
72. I must here state that I have found some difficulty in estimating the normal
standard of fertility for the several unions of this species, as the results
differ much during successive years, and the seeds vary so greatly in size that
it is hard to decide which ought to be considered good. In order to avoid over-
estimating the infertility of the several illegitimate unions, I have taken the
normal standard as low as possible.
From the foregoing twenty-seven illegitimate plants, fertilised with their own-
form pollen, twenty-five seedling grandchildren were raised; and these were all
long-styled; so that from the two illegitimate generations fifty-two plants were
raised, and all without exception proved long-styled. These grandchildren grew
vigorously, and soon exceeded in height two other lots of illegitimate seedlings
of different parentage and one lot of equal-styled seedlings presently to be
described. Hence I expected that they would have turned out highly ornamental
plants; but when they flowered, they seemed, as my gardener remarked, to have
gone back to the wild state; for the petals were pale-coloured, narrow,
sometimes not touching each other, flat, generally deeply notched in the middle,
but not flexuous on the margin, and with the yellow eye or centre conspicuous.
Altogether these flowers were strikingly different from those of their
progenitors; and this I think, can only be accounted for on the principle of
reversion. Most of the anthers on one plant were contabescent. Seventeen flowers
on the grandchildren were illegitimately fertilised with pollen taken from other
seedlings of the same lot, and produced fourteen capsules, containing on an
average 29.2 seeds; but they ought to have contained about 35 seeds. Fifteen
flowers legitimately fertilised with pollen from an illegitimate short-styled
plant (belonging to the lot next to be described) produced fourteen capsules,
containing an average of 46 seeds; they ought to have contained at least 50
seeds. Hence these grandchildren of illegitimate descent appear to have lost,
though only in a very slight degree, their full fertility.
We will now turn to the short-styled form: from a plant of this kind, fertilised
with its own-form pollen, I raised, during February 1862, eight seedlings, seven
of which were short-styled and one long-styled. They grew slowly, and never
attained to the full stature of ordinary plants; some of them flowered
precociously, and others late in the season. Four flowers on these short-styled
seedlings and four on the one long-styled seedling were illegitimately
fertilised with their own-form pollen and produced only three capsules,
containing on an average 23.6 seeds, with a maximum of 29; but we cannot judge
of their fertility from so few capsules; and I have greater doubts about the
normal standard for this union than about any other; but I believe that rather
above 25 seeds would be a fair estimate. Eight flowers on these same short-
styled plants, and the one long-styled illegitimate plant were reciprocally and
legitimately crossed; they produced five capsules, which contained an average of
28.6 seeds, with a maximum of 36. A reciprocal cross between legitimate plants
of the two forms would have yielded an average of at least 57 seeds, with a
possible maximum of 74 seeds; so that these illegitimate plants were sterile
when legitimately crossed.
I succeeded in raising from the above seven short-styled illegitimate plants,
fertilised with their own-form pollen, only six plants--grandchildren of the
first union. These, like their parents, were of low stature, and had so poor a
constitution that four died before flowering. With ordinary plants it has been a
rare event with me to have more than a single plant die out of a large lot. The
two grandchildren which lived and flowered were short-styled; and twelve of
their flowers were fertilised with their own-form pollen and produced twelve
capsules containing an average of 28.2 seeds; so that these two plants, though
belonging to so weakly a set, were rather more fertile than their parents, and
perhaps not in any degree sterile. Four flowers on the same two grandchildren
were legitimately fertilised by a long-styled illegitimate plant, and produced
four capsules, containing only 32.2 seeds instead of about 64 seeds, which is
the normal average for legitimate short-styled plants legitimately crossed.
By looking back, it will be seen that I raised at first from a short-styled
plant fertilised with its own-form pollen one long-styled and seven short-styled
illegitimate seedlings. These seedlings were legitimately intercrossed, and from
their seed fifteen plants were raised, grandchildren of the first illegitimate
union, and to my surprise all proved short-styled. Twelve short-styled flowers
borne by these grandchildren were illegitimately fertilised with pollen taken
from other plants of the same lot, and produced eight capsules which contained
an average of 21.8 seeds, with a maximum of 35. These figures are rather below
the normal standard for such a union. Six flowers were also legitimately
fertilised with pollen from an illegitimate long-styled plant and produced only
three capsules, containing on an average 23.6 seeds, with a maximum of 35. Such
a union in the case of a legitimate plant ought to have yielded an average of 64
seeds, with a possible maximum of 73 seeds.
SUMMARY ON THE TRANSMISSION OF FORM, CONSTITUTION, AND FERTILITY OF THE
ILLEGITIMATE OFFSPRING OF Primula Sinensis.
In regard to the long-styled plants, their illegitimate offspring, of which
fifty-two were raised in the course of two generations, were all long-styled.
(5/7. Dr. Hildebrand, who first called attention to this subject 'Botanische
Zeitung' 1864 page 5, raised from a similar illegitimate union seventeen plants,
of which fourteen were long-styled and three short-styled. From a short-styled
plant illegitimately fertilised with its own pollen he raised fourteen plants,
of which eleven were short-styled and three long-styled.) These plants grew
vigorously; but the flowers in one instance were small, appearing as if they had
reverted to the wild state. In the first illegitimate generation they were
perfectly fertile, and in the second their fertility was only very slightly
impaired. With respect to the short-styled plants, twenty-four out of twenty-
five of their illegitimate offspring were short-styled. They were dwarfed in
stature, and one lot of grandchildren had so poor a constitution that four out
of six plants perished before flowering. The two survivors, when illegitimately
fertilised with their own-form pollen, were rather less fertile than they ought
to have been; but their loss of fertility was clearly shown in a special and
unexpected manner, namely, when legitimately fertilised by other illegitimate
plants: thus altogether eighteen flowers were fertilised in this manner, and
yielded twelve capsules, which included on an average only 28.5 seeds, with a
maximum of 45. Now a legitimate short-styled plant would have yielded, when
legitimately fertilised, an average of 64 seeds, with a possible maximum of 74.
This particular kind of infertility will perhaps be best appreciated by a
simile: we may assume that with mankind six children would be born on an average
from an ordinary marriage; but that only three would be born from an incestuous
marriage. According to the analogy of Primula Sinensis, the children of such
incestuous marriages, if they continued to marry incestuously, would have their
sterility only slightly increased; but their fertility would not be restored by
a proper marriage; for if two children, both of incestuous origin, but in no
degree related to each other, were to marry, the marriage would of course be
strictly legitimate, nevertheless they would not give birth to more than half
the full and proper number of children.
[EQUAL-STYLED VARIETY OF Primula Sinensis.
As any variation in the structure of the reproductive organs, combined with
changed function, is a rare event, the following cases are worth giving in
detail. My attention was first called to the subject by observing, in 1862, a
long-styled plant, descended from a self-fertilised long-styled parent, which
had some of its flowers in an anomalous state, namely, with the stamens placed
low down in the corolla as in the ordinary long-styled form, but with the
pistils so short that the stigmas stood on a level with the anthers. These
stigmas were nearly as globular and as smooth as in the short-styled form,
instead of being elongated and rough as in the long-styled form. Here, then, we
have combined in the same flower, the short stamens of the long-styled form with
a pistil closely resembling that of the short-styled form. But the structure
varied much even on the same umbel: for in two flowers the pistil was
intermediate in length between that of the long and that of the short-styled
form, with the stigma elongated as in the former, and smooth as in the latter;
and in three other flowers the structure was in all respects like that of the
long-styled form. These modifications appeared to me so remarkable that I
fertilised eight of the flowers with their own pollen, and obtained five
capsules, which contained on an average 43 seeds; and this number shows that the
flowers had become abnormally fertile in comparison with those of ordinary long-
styled plants when self-fertilised. I was thus led to examine the plants in
several small collections, and the result showed that the equal-styled variety
was not rare.
TABLE 5.31. Primula Sinensis. Preponderance of long-styled over the short-styled
form.
Column 1: Name of owner or place.
Column 2: Long-styled form.
Column 3: Short-styled form.
Column 4: equal-styled variety.
Mr. Horwood : 0 : 0 : 17.
Mr. Duck : 20 : 0 : 9.
Baston : 30 : 18 : 15.
Chichester : 12 : 9 : 2.
Holwood : 42 : 12 : 0.
High Elms : 16 : 0 : 0.
Westerham : 1 : 5 : 0.
My own plants from purchased seeds : 13 : 7 : 0.
Total : 134 : 51 : 43.
In a state of nature the long and short-styled forms would no doubt occur in
nearly equal numbers, as I infer from the analogy of the other heterostyled
species of Primula, and from having raised the two forms of the present species
in exactly the same number from flowers which had been LEGITIMATELY crossed. The
preponderance in Table 5.31 of the long-styled form over the short-styled (in
the proportion of 134 to 51) results from gardeners generally collecting seed
from self-fertilised flowers; and the long-styled flowers produce spontaneously
much more seed (as shown in the first chapter) than the short-styled, owing to
the anthers of the long-styled form being placed low down in the corolla, so
that, when the flowers fall off, the anthers are dragged over the stigma; and we
now also know that long-styled plants, when self-fertilised, very generally
reproduce long-styled offspring. From the consideration of this table, it
occurred to me in the year 1862, that almost all the plants of the Chinese
primrose cultivated in England would sooner or later become long-styled or
equal-styled; and now, at the close of 1876, I have had five small collections
of plants examined, and almost all consisted of long-styled, with some more or
less well-characterised equal-styled plants, but with not one short-styled.
With respect to the equal-styled plants in the table, Mr. Horwood raised from
purchased seeds four plants, which he remembered were certainly not long-styled,
but either short or equal-styled, probably the latter. These four plants were
kept separate and allowed to fertilise themselves; from their seed the seventeen
plants in the table were raised, all of which proved equal-styled. The stamens
stood low down in the corolla as in the long-styled form; and the stigmas, which
were globular and smooth, were either completely surrounded by the anthers, or
stood close above them. My son William made drawings for me, by the aid of the
camera, of the pollen of one of the above equal-styled plants; and, in
accordance with the position of the stamens, the grains resembled in their small
size those of the long-styled form. He also examined pollen from two equal-
styled plants at Southampton; and in both of them the grains differed extremely
in size in the same anthers, a large number being small and shrivelled, whilst
many were fully as large as those of the short-styled form and rather more
globular. It is probable that the large size of these grains was due, not to
their having assumed the character of the short-styled form, but to monstrosity;
for Max Wichura has observed pollen-grains of monstrous size in certain hybrids.
The vast number of the small shrivelled grains in the above two cases explains
the fact that, though equal-styled plants are generally fertile in a high
degree, yet some of them yield few seeds. I may add that my son compared, in
1875, the grains from two white-flowered plants, in both of which the pistil
projected above the anthers, but neither were properly long-styled or equal-
styled; and in the one in which the stigma projected most, the grains were in
diameter to those in the other plant, in which the stigma projected less, as 100
to 88; whereas the difference between the grains from perfectly characterised
long-styled and short-styled plants is as 100 to 57. So that these two plants
were in an intermediate condition. To return to the 17 plants in the first line
of Table 5.31: from the relative position of their stigmas and anthers, they
could hardly fail to fertilise themselves; and accordingly four of them
spontaneously yielded no less than 180 capsules; of these Mr. Horwood selected
eight fine capsules for sowing; and they included on an average 54.8 seeds, with
a maximum of 72. He gave me thirty other capsules, taken by hazard, of which
twenty-seven contained good seeds, averaging 35.5, with a maximum of 70; but if
six poor capsules, each with less than 13 seeds, be excluded, the average rises
to 42.5. These are higher numbers than could be expected from either well-
characterised form if self-fertilised; and this high degree of fertility accords
with the view that the male organs belonged to one form, and the female organs
partially to the other form; so that a self-union in the case of the equal-
styled variety is in fact a legitimate union.
The seed saved from the above seventeen self-fertilised equal-styled plants
produced sixteen plants, which all proved equal-styled, and resembled their
parents in all the above-specified respects. The stamens, however, in one plant
were seated higher up the tube of the corolla than in the true long-styled form;
in another plant almost all the anthers were contabescent. These sixteen plants
were the grandchildren of the four original plants, which it is believed were
equal-styled; so that this abnormal condition was faithfully transmitted,
probably through three, and certainly through two generations. The fertility of
one of these grandchildren was carefully observed: six flowers were fertilised
with pollen from the same flower, and produced six capsules, containing on an
average 68 seeds, with a maximum of 82, and a minimum of 40. Thirteen capsules
spontaneously self-fertilised yielded an average of 53.2 seeds, with the
astonishing maximum in one of 97 seeds. In no legitimate union has so high an
average as 68 seeds been observed by me, or nearly so high a maximum as 82 and
97. These plants, therefore, not only have lost their proper heterostyled
structure and peculiar functional powers, but have acquired an abnormal grade of
fertility--unless, indeed, their high fertility may be accounted for by the
stigmas receiving pollen from the circumjacent anthers at exactly the most
favourable period.
With respect to Mr. Duck's lot in Table 5.31, seed was saved from a single
plant, of which the form was not observed, and this produced nine equal-styled
and twenty long-styled plants. The equal-styled resembled in all respects those
previously described; and eight of their capsules spontaneously self-fertilised
contained on an average 44.4 seeds, with a maximum of 61 and a minimum of 23. In
regard to the twenty long-styled plants, the pistil in some of the flowers did
not project quite so high as in ordinary long-styled flowers; and the stigmas,
though properly elongated, were smooth; so that we have here a slight approach
in structure to the pistil of the short-styled form. Some of these long-styled
plants also approached the equal-styled in function; for one of them produced no
less than fifteen spontaneously self-fertilised capsules, and of these eight
contained, on an average, 31.7 seeds, with a maximum of 61. This average would
be rather low for a long-styled plant artificially fertilised with its own
pollen, but is high for one spontaneously self-fertilised. For instance, thirty-
four capsules produced by the illegitimate grandchildren of a long-styled plant,
spontaneously self-fertilised, contained on an average only 9.1 seeds, with a
maximum of 46. Some seeds indiscriminately saved from the foregoing twenty-nine
equal-styled and long-styled plants produced sixteen seedlings, grandchildren of
the original plant belonging to Mr. Duck; and these consisted of fourteen equal-
styled and two long-styled plants; and I mention this fact as an additional
instance of the transmission of the equal-styled variety.
The third lot in Table 5.31, namely the Baston plants, are the last which need
be mentioned. The long and short-styled plants, and the fifteen equal-styled
plants, were descended from two distinct stocks. The latter were derived from a
single plant, which the gardener is positive was not long-styled; hence,
probably, it was equal-styled. In all these fifteen plants the anthers,
occupying the same position as in the long-styled form, closely surrounded the
stigma, which in one instance alone was slightly elongated. Notwithstanding this
position of the stigma, the flowers, as the gardener assured me, did not yield
many seeds; and this difference from the foregoing cases may perhaps have been
caused by the pollen being bad, as in some of the Southampton equal-styled
plants.]
CONCLUSIONS WITH RESPECT TO THE EQUAL-STYLED VARIETY OF P. Sinensis.
That this is a variation, and not a third or distinct form, as in the trimorphic
genera Lythrum and Oxalis, is clear; for we have seen its first appearance in
one out of a lot of illegitimate long-styled plants; and in the case of Mr.
Duck's seedlings, long-styled plants, only slightly deviating from the normal
state, as well as equal-styled plants were produced from the same self-
fertilised parent. The position of the stamens in their proper place low down in
the tube of the corolla, together with the small size of the pollen-grains,
show, firstly, that the equal-styled variety is a modification of the long-
styled form, and, secondly, that the pistil is the part which has varied most,
as indeed was obvious in many of the plants. This variation is of frequent
occurrence, and is strongly inherited when it has once appeared. It would,
however, have possessed little interest if it had consisted of a mere change of
structure; but this is accompanied by modified fertility. Its occurrence
apparently stands in close relation with the illegitimate birth of the parent
plant; but to this whole subject I shall hereafter recur.
[Primula auricula.
Although I made no experiments on the illegitimate offspring of this species, I
refer to it for two reasons:--First, because I have observed two equal-styled
plants in which the pistil resembled in all respects that of the long-styled
form, whilst the stamens had become elongated as in the short-styled form, so
that the stigma was almost surrounded by the anthers. The pollen-grains,
however, of the elongated stamens resembled in their small size those of the
shorter stamens proper to the long-styled form. Hence these plants have become
equal-styled by the increased length of the stamens, instead of, as with P.
Sinensis, by the diminished length of the pistil. Mr. J. Scott observed five
other plants in the same state, and he shows that one of them, when self-
fertilised, yielded more seed than an ordinary long- or short-styled form would
have done when similarly fertilised, but that it was far inferior in fertility
to either form when legitimately crossed. (5/8. 'Journal of the Proceedings of
the Linnean Society' 8 1864 page 91.) Hence it appears that the male and female
organs of this equal-styled variety have been modified in some special manner,
not only in structure but in functional powers. This, moreover, is shown by the
singular fact that both the long-styled and short-styled plants, fertilised with
pollen from the equal-styled variety, yield a lower average of seed than when
these two forms are fertilised with their own pollen.
The second point which deserves notice is that florists always throw away the
long-styled plants, and save seed exclusively from the short-styled form.
Nevertheless, as Mr. Scott was informed by a man who raises this species
extensively in Scotland, about one-fourth of the seedlings appear long-styled;
so that the short-styled form of the Auricula, when fertilised by its own
pollen, does not reproduce the same form in so large a proportion as in the case
of P. Sinensis. We may further infer that the short-styled form is not rendered
quite sterile by a long course of fertilisation with pollen of the same form:
but as there would always be some liability to an occasional cross with the
other form, we cannot tell how long self-fertilisation has been continued.
Primula farinosa.
Mr. Scott says that it is not at all uncommon to find equal-styled plants of
this heterostyled species. (5/9. 'Journal of the Proceedings of the Linnean
Society' 8 1864 page 115.) Judging from the size of the pollen-grains, these
plants owe their structure, as in the case of P. auricula, to the abnormal
elongation of the stamens of the long-styled form. In accordance with this view,
they yield less seed when crossed with the long-styled form than with the short-
styled. But they differ in an anomalous manner from the equal-styled plants of
P. auricula in being extremely sterile with their own pollen.
Primula elatior.
It was shown in the first chapter, on the authority of Herr Breitenbach, that
equal-styled flowers are occasionally found on this species whilst growing in a
state of nature; and this is the only instance of such an occurrence known to
me, with the exception of some wild plants of the Oxlip--a hybrid between P.
veris and vulgaris--which were equal-styled. Herr Breitenbach's case is
remarkable in another way; for equal-styled flowers were found in two instances
on plants which bore both long-styled and short-styled flowers. In every other
instance these two forms and the equal-styled variety have been produced by
distinct plants.]
Primula vulgaris, BRIT. FL.
VAR. acaulis OF LINN. AND P. acaulis OF JACQ.
VAR. RUBRA.
Mr. Scott states that this variety, which grew in the Botanic Garden in
Edinburgh, was quite sterile when fertilised with pollen from the common
primrose, as well as from a white variety of the same species, but that some of
the plants, when artificially fertilised with their own pollen, yielded a
moderate supply of seed. (5/10. 'Journal of the Proceedings of the Linnean
Society' 8 1864 page 98.) He was so kind as to send me some of these self-
fertilised seeds, from which I raised the plants immediately to be described. I
may premise that the results of my experiments on the seedlings, made on a large
scale, do not accord with those by Mr. Scott on the parent-plant.
First, in regard to the transmission of form and colour. The parent-plant was
long-styled, and of a rich purple colour. From the self-fertilised seed 23
plants were raised; of these 18 were purple of different shades, with two of
them a little streaked and freckled with yellow, thus showing a tendency to
reversion; and 5 were yellow, but generally with a brighter orange centre than
in the wild flower. All the plants were profuse flowerers. All were long-styled;
but the pistil varied a good deal in length even on the same plant, being rather
shorter, or considerably longer, than in the normal long-styled form; and the
stigmas likewise varied in shape. It is, therefore, probable that an equal-
styled variety of the primrose might be found on careful search; and I have
received two accounts of plants apparently in this condition. The stamens always
occupied their proper position low down in the corolla; and the pollen-grains
were of the small size proper to the long-styled form, but were mingled with
many minute and shrivelled grains. The yellow-flowered and the purple-flowered
plants of this first generation were fertilised under a net with their own
pollen, and the seed separately sown. From the former, 22 plants were raised,
and all were yellow and long-styled. From the latter or the purple-flowered
plants, 24 long-styled plants were raised, of which 17 were purple and 7 yellow.
In this last case we have an instance of reversion in colour, without the
possibility of any cross, to the grandparents or more distant progenitors of the
plants in question. Altogether 23 plants in the first generation and 46 in the
second generation were raised; and the whole of these 69 illegitimate plants
were long-styled!
Eight purple-flowered and two yellow-flowered plants of the first illegitimate
generation were fertilised in various ways with their own pollen and with that
of the common primrose; and the seeds were separately counted, but as I could
detect no difference in fertility between the purple and yellow varieties, the
results are run together in Table 5.32.
TABLE 5.32. Primula vulgaris.
Column 1: Nature of plant experimented on, and kind of union.
Column 2: Number of flowers fertilised.
Column 3: Number of capsules produced.
Column 4: Average Number of seeds per capsule.
Column 5: Maximum Number of seeds in any one capsule.
Column 6: Minimum Number of seeds in any one capsule.
Purple- and yellow-flowered illegitimate long-styled plants, ILLEGITIMATELY
fertilised with pollen from the same plant :
72 : 11 : 11.5 : 26 : 5.
Purple- and yellow-flowered illegitimate long-styled plants, ILLEGITIMATELY
fertilised with pollen from the common long-styled primrose :
72 : 39 : 31.4 : 62 : 3.
Or, if the ten poorest capsules, including less than 15 seeds, be rejected, we
get:
72 : 29 : 40.6 : 62 : 18.
Purple- and yellow-flowered illegitimate long-styled plants, LEGITIMATELY
fertilised with pollen from the common short-styled primrose :
26 : 18 : 36.4 : 60 : 9.
Or, if the two poorest capsules, including less than 15 seeds, be rejected, we
get:
26 : 16 : 41.2 : 60 : 15.
The long-styled form of the common primrose ILLEGITIMATELY fertilised with
pollen from the long-styled illegitimate purple- and yellow-flowered plants:
20 : 14 : 15.4 : 46 : 1.
Or, if the three poorest capsules be rejected, we get:
20 : 11 : 18.9 : 46 : 8.
The short-styled form of the common primrose LEGITIMATELY fertilised with pollen
from the long-styled illegitimate purple- and yellow-flowered plants:
10 : 6 : 30.5 : 61 : 6.
If we compare the figures in this table with those given in the first chapter,
showing the normal fertility of the common primrose, we shall see that the
illegitimate purple- and yellow-flowered varieties are very sterile. For
instance, 72 flowers were fertilised with their own pollen and produced only 11
good capsules; but by the standard they ought to have produced 48 capsules; and
each of these ought to have contained on an average 52.2 seeds, instead of only
11.5 seeds. When these plants were illegitimately and legitimately fertilised
with pollen from the common primrose, the average numbers were increased, but
were far from attaining the normal standards. So it was when both forms of the
common primrose were fertilised with pollen from these illegitimate plants; and
this shows that their male as well as their female organs were in a deteriorated
condition. The sterility of these plants was shown in another way, namely, by
their not producing any capsules when the access of all insects (except such
minute ones as Thrips) was prevented; for under these circumstances the common
long-styled primrose produces a considerable number of capsules. There can,
therefore, be no doubt that the fertility of these plants was greatly impaired.
The loss is not correlated with the colour of the flower; and it was to
ascertain this point that I made so many experiments. As the parent-plant
growing in Edinburgh was found by Mr. Scott to be in a high degree sterile, it
may have transmitted a similar tendency to its offspring, independently of their
illegitimate birth. I am, however, inclined to attribute some weight to the
illegitimacy of their descent, both from the analogy of other cases, and more
especially from the fact that when the plants were LEGITIMATELY fertilised with
pollen of the common primrose they yielded an average, as may be seen in the
table, of only 5 more seeds than when ILLEGITIMATELY fertilised with the same
pollen. Now we know that it is eminently characteristic of the illegitimate
offspring of Primula Sinensis that they yield but few more seeds when
legitimately fertilised than when fertilised with their own-form pollen.
Primula veris, Brit. Fl.
Var. officinalis of Linn., P. officinalis OF Jacq.
Seeds from the short-styled form of the cowslip fertilised with pollen from the
same form germinate so badly that I raised from three successive sowings only
fourteen plants, which consisted of nine short-styled and five long-styled
plants. Hence the short-styled form of the cowslip, when self-fertilised, does
not transmit the same form nearly so truly as does that of P. Sinensis. From the
long-styled form, always fertilised with its own-form pollen, I raised in the
first generation three long-styled plants,--from their seed 53 long-styled
grandchildren,--from their seed 4 long-styled great-grandchildren,--from their
seed 20 long-styled great-great-grandchildren,--and lastly, from their seed 8
long-styled and 2 short-styled great-great-great-grandchildren. In this last
generation short-styled plants appeared for the first time in the course of the
six generations,--the parent long-styled plant which was fertilised with pollen
from another plant of the same form being counted as the first generation. Their
appearance may be attributed to atavism. From two other long-styled plants,
fertilised with their own-form pollen, 72 plants were raised, which consisted of
68 long-styled and 4 short-styled. So that altogether 162 plants were raised
from illegitimately fertilised long-styled cowslips, and these consisted of 156
long-styled and 6 short-styled plants.
We will now turn to the fertility and powers of growth possessed by the
illegitimate plants. From a short-styled plant, fertilised with its own-form
pollen, one short-styled and two long-styled plants, and from a long-styled
plant similarly fertilised three long-styled plants were at first raised. The
fertility of these six illegitimate plants was carefully observed; but I must
premise that I cannot give any satisfactory standard of comparison as far as the
number of the seeds is concerned; for though I counted the seeds of many
legitimate plants fertilised legitimately and illegitimately, the number varied
so greatly during successive seasons that no one standard will serve well for
illegitimate unions made during different seasons. Moreover the seeds in the
same capsule frequently differ so much in size that it is scarcely possible to
decide which ought to be counted as good seed. There remains as the best
standard of comparison the proportional number of fertilised flowers which
produce capsules containing any seed.
First, for the one illegitimate short-styled plant. In the course of three
seasons 27 flowers were illegitimately fertilised with pollen from the same
plant, and they yielded only a single capsule, which, however, contained a
rather large number of seeds for a union of this nature, namely, 23. As a
standard of comparison I may state that during the same three seasons 44 flowers
borne by legitimate short-styled plants were self-fertilised, and yielded 26
capsules; so that the fact of the 27 flowers on the illegitimate plant having
produced only one capsule proves how sterile it was. To show that the conditions
of life were favourable, I will add that numerous plants of this and other
species of Primula all produced an abundance of capsules whilst growing close by
in the same soil with the present and following plants. The sterility of the
above illegitimate short-styled plant depended on both the male and female
organs being in a deteriorated condition. This was manifestly the case with the
pollen; for many of the anthers were shrivelled or contabescent. Nevertheless
some of the anthers contained pollen, with which I succeeded in fertilising some
flowers on the illegitimate long-styled plants immediately to be described. Four
flowers on this same short-styled plant were likewise LEGITIMATELY fertilised
with pollen from one of the following long-styled plants; but only one capsule
was produced, containing 26 seeds; and this is a very low number for a
legitimate union.
With respect to the five illegitimate long-styled plants of the first
generation, derived from the above self-fertilised short-styled and long-styled
parents, their fertility was observed during the same three years. These five
plants, when self-fertilised, differed considerably from one another in their
degree of fertility, as was the case with the illegitimate long-styled plants of
Lythrum salicaria; and their fertility varied much according to the season. I
may premise, as a standard of comparison, that during the same years 56 flowers
on legitimate long-styled plants of the same age and grown in the same soil,
were fertilised with their own pollen, and yielded 27 capsules; that is, 48 per
cent. On one of the five illegitimate long-styled plants 36 flowers were self-
fertilised in the course of the three years, but they did not produce a single
capsule. Many of the anthers on this plant were contabescent; but some seemed to
contain sound pollen. Nor were the female organs quite impotent; for I obtained
from a LEGITIMATE cross one capsule with good seed. On a second illegitimate
long-styled plant 44 flowers were fertilised during the same years with their
own pollen, but they produced only a single capsule. The third and fourth plants
were in a very slight degree more productive. The fifth and last plant was
decidedly more fertile; for 42 self-fertilised flowers yielded 11 capsules.
Altogether, in the course of the three years, no less than 160 flowers on these
five illegitimate long-styled plants were fertilised with their own pollen, but
they yielded only 22 capsules. According to the standard above given, they ought
to have yielded 80 capsules. These 22 capsules contained on an average 15.1
seeds. I believe, subject to the doubts before specified, that with legitimate
plants the average number from a union of this nature would have been above 20
seeds. Twenty-four flowers on these same five illegitimate long-styled plants
were legitimately fertilised with pollen from the above-described illegitimate
short-styled plant, and produced only 9 capsules, which is an extremely small
number for a legitimate union. These 9 capsules, however, contained an average
of 38 apparently good seeds, which is as large a number as legitimate plants
sometimes yield. But this high average was almost certainly false; and I mention
the case for the sake of showing the difficulty of arriving at a fair result;
for this average mainly depended on two capsules containing the extraordinary
numbers of 75 and 56 seeds; these seeds, however, though I felt bound to count
them, were so poor that, judging from trials made in other cases, I do not
suppose that one would have germinated; and therefore they ought not to have
been included. Lastly, 20 flowers were legitimately fertilised with pollen from
a legitimate plant, and this increased their fertility; for they produced 10
capsules. Yet this is but a very small proportion for a legitimate union.
There can, therefore, be no doubt that these five long-styled plants and the one
short-styled plant of the first illegitimate generation were extremely sterile.
Their sterility was shown, as in the case of hybrids, in another way, namely, by
their flowering profusely, and especially by the long endurance of the flowers.
For instance, I fertilised many flowers on these plants, and fifteen days
afterwards (namely on March 22nd) I fertilised numerous long-styled and short-
styled flowers on common cowslips growing close by. These latter flowers, on
April 8th, were withered, whilst most of the illegitimate flowers remained quite
fresh for several days subsequently; so that some of these illegitimate plants,
after being fertilised, remained in full bloom for above a month.
We will now turn to the fertility of the 53 illegitimate long-styled
grandchildren, descended from the long-styled plant which was first fertilised
with its own pollen. The pollen in two of these plants included a multitude of
small and shrivelled grains. Nevertheless they were not very sterile; for 25
flowers, fertilised with their own pollen, produced 15 capsules, containing an
average of 16.3 seeds. As already stated, the probable average with legitimate
plants for a union of this nature is rather above 20 seeds. These plants were
remarkably healthy and vigorous, as long as they were kept under highly
favourable conditions in pots in the greenhouse; and such treatment greatly
increases the fertility of the cowslip. When these same plants were planted
during the next year (which, however, was an unfavourable one), out of doors in
good soil, 20 self-fertilised flowers produced only 5 capsules, containing
extremely few and wretched seeds.
Four long-styled great-grandchildren were raised from the self-fertilised
grandchildren, and were kept under the same highly favourable conditions in the
greenhouse; 10 of their flowers were fertilised with own-form pollen and yielded
the large proportion of 6 capsules, containing on an average 18.7 seeds. From
these seeds 20 long-styled great-great-grandchildren were raised, which were
likewise kept in the greenhouse. Thirty of their flowers were fertilised with
their own pollen and yielded 17 capsules, containing on an average no less than
32, mostly fine seeds. It appears, therefore, that the fertility of these plants
of the fourth illegitimate generation, as long as they were kept under highly
favourable conditions, had not decreased, but had rather increased. The result,
however, was widely different when they were planted out of doors in good soil,
where other cowslips grew vigorously and were completely fertile; for these
illegitimate plants now became much dwarfed in stature and extremely sterile,
notwithstanding that they were exposed to the visits of insects, and must have
been legitimately fertilised by the surrounding legitimate plants. A whole row
of these plants of the fourth illegitimate generation, thus freely exposed and
legitimately fertilised, produced only 3 capsules, containing on an average only
17 seeds. During the ensuing winter almost all these plants died, and the few
survivors were miserably unhealthy, whilst the surrounding legitimate plants
were not in the least injured.
The seeds from the great-great-grandchildren were sown, and 8 long-styled and 2
short-styled plants of the fifth illegitimate generation raised. These whilst
still in the greenhouse produced smaller leaves and shorter flower-stalks than
some legitimate plants with which they grew in competition; but it should be
observed that the latter were the product of a cross with a fresh stock,--a
circumstance which by itself would have added much to their vigour. (5/11. For
full details of this experiment, see my 'Effects of Cross and Self-
fertilisation' 1876 page 220.) When these illegitimate plants were transferred
to fairly good soil out of doors, they became during the two following years
much more dwarfed in stature and produced very few flower-stems; and although
they must have been legitimately fertilised by insects, they yielded capsules,
compared with those produced by the surrounding legitimate plants, in the ratio
only of 5 to 100! It is therefore certain that illegitimate fertilisation,
continued during successive generations, affects the powers of growth and
fertility of P. veris to an extraordinary degree; more especially when the
plants are exposed to ordinary conditions of life, instead of being protected in
a greenhouse.
[EQUAL-STYLED RED VARIETY OF Primula veris.
Mr. Scott has described a plant of this kind growing in the Botanic Garden of
Edinburgh. (5/12. 'Proceedings of the Linnean Society' volume 8 1864 page 105.)
He states that it was highly self-fertile, although insects were excluded; and
he explains this fact by showing, first, that the anthers and stigma are in
close apposition, and that the stamens in length, position and size of their
pollen-grains resemble those of the short-styled form, whilst the pistil
resembles that of the long-styled form both in length and in the structure of
the stigma. Hence the self-union of this variety is, in fact, a legitimate
union, and consequently is highly fertile. Mr. Scott further states that this
variety yielded very few seeds when fertilised by either the long- or short-
styled common cowslip, and, again, that both forms of the latter, when
fertilised by the equal-styled variety, likewise produced very few seeds. But
his experiments with the cowslip were few, and my results do not confirm his in
any uniform manner.
I raised twenty plants from self-fertilised seed sent me by Mr. Scott; and they
all produced red flowers, varying slightly in tint. Of these, two were strictly
long-styled both in structure and in function; for their reproductive powers
were tested by crosses with both forms of the common cowslip. Six plants were
equal-styled; but on the same plant the pistil varied a good deal in length
during different seasons. This was likewise the case, according to Mr. Scott,
with the parent-plant. Lastly, twelve plants were in appearance short-styled;
but they varied much more in the length of their pistils than ordinary short-
styled cowslips, and they differed widely from the latter in their powers of
reproduction. Their pistils had become short-styled in structure, whilst
remaining long-styled in function. Short-styled cowslips, when insects are
excluded, are extremely barren: for instance, on one occasion six fine plants
produced only about 50 seeds (that is, less than the product of two good
capsules), and on another occasion not a single capsule. Now, when the above
twelve apparently short-styled seedlings were similarly treated, nearly all
produced a great abundance of capsules, containing numerous seeds, which
germinated remarkably well. Moreover three of these plants, which during the
first year were furnished with quite short pistils, on the following year
produced pistils of extraordinary length. The greater number, therefore, of
these short-styled plants could not be distinguished in function from the equal-
styled variety. The anthers in the six equal-styled and in the apparently twelve
short-styled plants were seated high up in the corolla, as in the true short-
styled cowslip; and the pollen-grains resembled those of the same form in their
large size, but were mingled with a few shrivelled grains. In function this
pollen was identical with that of the short-styled cowslip; for ten long-styled
flowers of the common cowslip, legitimately fertilised with pollen from a true
equal-styled variety, produced six capsules, containing on an average 34.4
seeds; whilst seven capsules on a short-styled cowslip illegitimately fertilised
with pollen from the equal-styled variety, yielded an average of only 14.5
seeds.
As the equal-styled plants differ from one another in their powers of
reproduction, and as this is an important subject, I will give a few details
with respect to five of them. First, an equal-styled plant, protected from
insects (as was done in all the following cases, with one stated exception),
spontaneously produced numerous capsules, five of which gave an average of 44.8
seeds, with a maximum in one capsule of 57. But six capsules, the product of
fertilisation with pollen from a short-styled cowslip (and this is a legitimate
union), gave an average of 28.5 seeds, with a maximum of 49; and this is a much
lower average than might have been expected. Secondly, nine capsules from
another equal-styled plant, which had not been protected from insects, but
probably was self-fertilised, gave an average of 45.2 seeds, with a maximum of
58. Thirdly, another plant which had a very short pistil in 1865, produced
spontaneously many capsules, six of which contained an average of 33.9 seeds,
with a maximum of 38. In 1866 this same plant had a pistil of wonderful length;
for it projected quite above the anthers, and the stigma resembled that of the
long-styled form. In this condition it produced spontaneously a vast number of
fine capsules, six of which contained almost exactly the same average number as
before, namely 34.3, with a maximum of 38. Four flowers on this plant,
legitimately fertilised with pollen from a short-styled cowslip, yielded
capsules with an average of 30.2 seeds. Fourthly another short-styled plant
spontaneously produced in 1865 an abundance of capsules, ten of which contained
an average of 35.6 seeds, with a maximum of 54. In 1866 this same plant had
become in all respects long-styled, and ten capsules gave almost exactly the
same average as before, namely 35.1 seeds, with a maximum of 47. Eight flowers
on this plant, legitimately fertilised with pollen from a short-styled cowslip,
produced six capsules, with the high average of 53 seeds, and the high maximum
of 67. Eight flowers were also fertilised with pollen from a long-styled cowslip
(this being an illegitimate union), and produced seven capsules, containing an
average of 24.4 seeds, with a maximum of 32. The fifth and last plant remained
in the same condition during both years: it had a pistil rather longer than that
of the true short-styled form, with the stigma smooth, as it ought to be in this
form, but abnormal in shape, like a much-elongated inverted cone. It produced
spontaneously many capsules, five of which, in 1865, gave an average of only
15.6 seeds; and in 1866 ten capsules still gave an average only a little higher,
namely of 22.1, with a maximum of 30. Sixteen flowers were fertilised with
pollen from a long-styled cowslip, and produced 12 capsules, with an average of
24.9 seeds, and a maximum of 42. Eight flowers were fertilised with pollen from
a short-styled cowslip, but yielded only two capsules, containing 18 and 23
seeds. Hence this plant, in function and partially in structure, was in an
almost exactly intermediate state between the long-styled and short-styled form,
but inclining towards the short-styled; and this accounts for the low average of
seeds which it produced when spontaneously self-fertilised.
The foregoing five plants thus differ much from one another in the nature of
their fertility. In two individuals a great difference in the length of the
pistil during two succeeding years made no difference in the number of seeds
produced. As all five plants possessed the male organs of the short-styled form
in a perfect state, and the female organs of the long-styled form in a more or
less complete state, they spontaneously produced a surprising number of
capsules, which generally contained a large average of remarkably fine seeds.
With ordinary cowslips LEGITIMATELY FERTILISED, I once obtained from plants
cultivated in the greenhouse the high average, from seven capsules, of 58.7
seeds, with a maximum in one capsule of 87 seeds; but from plants grown out of
doors I never obtained a higher average than 41 seeds. Now two of the equal-
styled plants, grown out of doors and spontaneously SELF-FERTILISED, gave
averages of 44 and 45 seeds; but this high fertility may perhaps be in part
attributed to the stigma receiving pollen from the surrounding anthers at
exactly the right period. Two of these plants, fertilised with pollen from a
short-styled cowslip (and this in fact is a legitimate union), gave a lower
average than when self-fertilised. On the other hand, another plant, when
similarly fertilised by a cowslip, yielded the unusually high average of 53
seeds, with a maximum of 67. Lastly, as we have just seen, one of these plants
was in an almost exactly intermediate condition in its female organs between the
long- and short-styled forms, and consequently, when self-fertilised, yielded a
low average of seed. If we add together all the experiments which I made on the
equal-styled plants, 41 spontaneously self-fertilised capsules (insects having
been excluded) gave an average of 34 seeds, which is exactly the same number as
the parent-plant yielded in Edinburgh. Thirty-four flowers, fertilised with
pollen from the short-styled cowslip (and this is an analogous union), produced
17 capsules, containing an average of 33.8 seeds. It is a rather singular
circumstance, for which I cannot account, that 20 flowers, artificially
fertilised on one occasion with pollen from the same plants yielded only ten
capsules, containing the low average of 26.7 seeds.
As bearing on inheritance, it may be added that 72 seedlings were raised from
one of the red-flowered, strictly equal-styled, self-fertilised plants descended
from the similarly characterised Edinburgh plant. These 72 plants were therefore
grandchildren of the Edinburgh plant, and they all bore, as in the first
generation, red flowers, with the exception of one plant, which reverted in
colour to the common cowslip. In regard to structure, nine plants were truly
long-styled and had their stamens seated low down in the corolla in the proper
position; the remaining 63 plants were equal-styled, though the stigma in about
a dozen of them stood a little below the anthers. We thus see that the anomalous
combination in the same flower, of the male and female sexual organs which
properly exist in the two distinct forms, was inherited with much force. Thirty-
six seedlings were also raised from long and short-styled common cowslips,
crossed with pollen from the equal-styled variety. Of these plants one alone was
equal-styled, 20 were short-styled, but with the pistil in three of them rather
too long, and the remaining 15 were long-styled. In this case we have an
illustration of the difference between simple inheritance and prepotency of
transmission; for the equal-styled variety, when self-fertilised, transmits its
character, as we have just seen, with much force, but when crossed with the
common cowslip cannot withstand the greater power of transmission of the latter.
PULMONARIA.
I have little to say on this genus. I obtained seeds of P. officinalis from a
garden where the long-styled form alone grew, and raised 11 seedlings, which
were all long-styled. These plants were named for me by Dr. Hooker. They
differed, as has been shown, from the plants belonging to this species which in
Germany were experimented on by Hildebrand (5/13. 'Botanische Zeitung' 1865 page
13.); for he found that the long-styled form was absolutely sterile with its own
pollen, whilst my long-styled seedlings and the parent-plants yielded a fair
supply of seed when self-fertilised. Plants of the long-styled form of
Pulmonaria angustifolia were, like Hildebrand's plants, absolutely sterile with
their own pollen, so that I could never procure a single seed. On the other
hand, the short-styled plants of this species, differently from those of P.
officinalis, were fertile with their own pollen in a quite remarkable degree for
a heterostyled plant. From seeds carefully self-fertilised I raised 18 plants,
of which 13 proved short-styled and 5 long-styled.
Polygonum fagopyrum.
From flowers on long-styled plants fertilised illegitimately with pollen from
the same plant, 49 seedlings were raised, and these consisted of 45 long-styled
and 4 short-styled. From flowers on short-styled plants illegitimately
fertilised with pollen from the same plant 33 seedlings were raised, and these
consisted of 20 short-styled and 13 long-styled. So that the usual rule of
illegitimately fertilised long-styled plants tending much more strongly than
short-styled plants to reproduce their own form here holds good. The
illegitimate plants derived from both forms flowered later than the legitimate,
and were to the latter in height as 69 to 100. But as these illegitimate plants
were descended from parents fertilised with their own pollen, whilst the
legitimate plants were descended from parents crossed with pollen from a
distinct individual, it is impossible to know how much of their difference in
height and period of flowering, is due to the illegitimate birth of the one set,
and how much to the other set being the product of a cross between distinct
plants.]
CONCLUDING REMARKS ON THE ILLEGITIMATE OFFSPRING OF HETEROSTYLED TRIMORPHIC AND
DIMORPHIC PLANTS.
It is remarkable how closely and in how many points illegitimate unions between
the two or three forms of the same heterostyled species, together with their
illegitimate offspring, resemble hybrid unions between distinct species together
with their hybrid offspring. In both cases we meet with every degree of
sterility, from very slightly lessened fertility to absolute barrenness, when
not even a single seed-capsule is produced. In both cases the facility of
effecting the first union is much influenced by the conditions to which the
plants are exposed. (5/14. This has been remarked by many experimentalists in
effecting crosses between distinct species; and in regard to illegitimate unions
I have given in the first chapter a striking illustration in the case of Primula
veris.) Both with hybrids and illegitimate plants the innate degree of sterility
is highly variable in plants raised from the same mother-plant. In both cases
the male organs are more plainly affected than the female; and we often find
contabescent anthers enclosing shrivelled and utterly powerless pollen-grains.
The more sterile hybrids, as Max Wichura has well shown, are sometimes much
dwarfed in stature, and have so weak a constitution that they are liable to
premature death (5/15. 'Die Bastardbefruchtung im Pflanzenreich' 1865.); and we
have seen exactly parallel cases with the illegitimate seedlings of Lythrum and
Primula. Many hybrids are the most persistent and profuse flowerers, as are some
illegitimate plants. When a hybrid is crossed by either pure parent-form, it is
notoriously much more fertile than when crossed inter se or by another hybrid;
so when an illegitimate plant is fertilised by a legitimate plant, it is more
fertile than when fertilised inter se or by another illegitimate plant. When two
species are crossed and they produce numerous seeds, we expect as a general rule
that their hybrid offspring will be moderately fertile; but if the parent
species produce extremely few seeds, we expect that the hybrids will be very
sterile. But there are marked exceptions, as shown by Gartner, to these rules.
So it is with illegitimate unions and illegitimate offspring. Thus the mid-
styled form of Lythrum salicaria, when illegitimately fertilised with pollen
from the longest stamens of the short-styled form, produced an unusual number of
seeds; and their illegitimate offspring were not at all, or hardly at all,
sterile. On the other hand, the illegitimate offspring from the long-styled
form, fertilised with pollen from the shortest stamens of the same form, yielded
few seeds, and the illegitimate offspring thus produced were very sterile; but
they were more sterile than might have been expected relatively to the
difficulty of effecting the union of the parent sexual elements. No point is
more remarkable in regard to the crossing of species than their unequal
reciprocity. Thus species A will fertilise B with the greatest ease; but B will
not fertilise A after hundreds of trials. We have exactly the same case with
illegitimate unions; for the mid-styled Lythrum salicaria was easily fertilised
by pollen from the longest stamens of the short-styled form, and yielded many
seeds; but the latter form did not yield a single seed when fertilised by the
longest stamens of the mid-styled form.
Another important point is prepotency. Gartner has shown that when a species is
fertilised with pollen from another species, if it be afterwards fertilised with
its own pollen, or with that of the same species, this is so prepotent over the
foreign pollen that the effect of the latter, though placed on the stigma some
time previously, is entirely destroyed. Exactly the same thing occurs with the
two forms of a heterostyled species. Thus several long-styled flowers of Primula
veris were fertilised illegitimately with pollen from another plant of the same
form, and twenty-four hours afterwards legitimately with pollen from a short-
styled dark-red polyanthus which is a variety of P. veris; and the result was
that every one of the thirty seedlings thus raised bore flowers more or less
red, showing plainly how prepotent the legitimate pollen from a short-styled
plant was over the illegitimate pollen from a long-styled plant.
In all the several foregoing points the parallelism is wonderfully close between
the effects of illegitimate and hybrid fertilisation. It is hardly an
exaggeration to assert that seedlings from an illegitimately fertilised
heterostyled plant are hybrids formed within the limits of one and the same
species. This conclusion is important, for we thus learn that the difficulty in
sexually uniting two organic forms and the sterility of their offspring, afford
no sure criterion of so-called specific distinctness. If any one were to cross
two varieties of the same form of Lythrum or Primula for the sake of
ascertaining whether they were specifically distinct, and he found that they
could be united only with some difficulty, that their offspring were extremely
sterile, and that the parents and their offspring resembled in a whole series of
relations crossed species and their hybrid offspring, he might maintain that his
varieties had been proved to be good and true species; but he would be
completely deceived. In the second place, as the forms of the same trimorphic or
dimorphic heterostyled species are obviously identical in general structure,
with the exception of the reproductive organs, and as they are identical in
general constitution (for they live under precisely the same conditions), the
sterility of their illegitimate unions and that of their illegitimate offspring,
must depend exclusively on the nature of the sexual elements and on their
incompatibility for uniting in a particular manner. And as we have just seen
that distinct species when crossed resemble in a whole series of relations the
forms of the same species when illegitimately united, we are led to conclude
that the sterility of the former must likewise depend exclusively on the
incompatible nature of their sexual elements, and not on any general difference
in constitution or structure. We are, indeed, led to this same conclusion by the
impossibility of detecting any differences sufficient to account for certain
species crossing with the greatest ease, whilst other closely allied species
cannot be crossed, or can be crossed only with extreme difficulty. We are led to
this conclusion still more forcibly by considering the great difference which
often exists in the facility of crossing reciprocally the same two species; for
it is manifest in this case that the result must depend on the nature of the
sexual elements, the male element of the one species acting freely on the female
element of the other, but not so in a reversed direction. And now we see that
this same conclusion is independently and strongly fortified by the
consideration of the illegitimate unions of trimorphic and dimorphic
heterostyled plants. In so complex and obscure a subject as hybridism it is no
slight gain to arrive at a definite conclusion, namely, that we must look
exclusively to functional differences in the sexual elements, as the cause of
the sterility of species when first crossed and of their hybrid offspring. It
was this consideration which led me to make the many observations recorded in
this chapter, and which in my opinion make them worthy of publication.
CHAPTER VI.
CONCLUDING REMARKS ON HETEROSTYLED PLANTS.
The essential character of heterostyled plants.
Summary of the differences in fertility between legitimately and illegitimately
fertilised plants.
Diameter of the pollen-grains, size of anthers and structure of stigma in the
different forms.
Affinities of the genera which include heterostyled species.
Nature of the advantages derived from heterostylism.
The means by which plants became heterostyled.
Transmission of form.
Equal-styled varieties of heterostyled plants.
Final remarks.
In the foregoing chapters all the heterostyled plants known to me have been more
or less fully described. Several other cases have been indicated, especially by
Professor Asa Gray and Kuhn, in which the individuals of the same species differ
in the length of their stamens and pistils (6/1. Asa Gray 'American Journal of
Science' 1865 page 101 and elsewhere as already referred to. Kuhn 'Botanische
Zeitung' 1867 page 67.); but as I have been often deceived by this character
taken alone, it seems to me the more prudent course not to rank any species as
heterostyled, unless we have evidence of more important differences between the
forms, as in the diameter of the pollen-grains, or in the structure of the
stigma. The individuals of many ordinary hermaphrodite plants habitually
fertilise one another, owing to their male and female organs being mature at
different periods, or to the structure of the parts, or to self-sterility, etc.;
and so it is with many hermaphrodite animals, for instance, land-snails or
earth-worms; but in all these cases any one individual can fully fertilise or be
fertilised by any other individual of the same species. This is not so with
heterostyled plants: a long-styled, mid-styled or short-styled plant cannot
fully fertilise or be fertilised by any other individual, but only by one
belonging to another form. Thus the essential character of plants belonging to
the heterostyled class is that the individuals are divided into two or three
bodies, like the males and females of dioecious plants or of the higher animals,
which exist in approximately equal numbers and are adapted for reciprocal
fertilisation. The existence, therefore, of two or three bodies of individuals,
differing from one another in the above more important characteristics, offers
by itself good evidence that the species is heterostyled. But absolutely
conclusive evidence can be derived only from experiments, and by finding that
pollen must be applied from the one form to the other in order to ensure
complete fertility.
In order to show how much more fertile each form is when legitimately fertilised
with pollen from the other form (or in the case of trimorphic species, with the
proper pollen from one of the two other forms) than when illegitimately
fertilised with its own-form pollen, I will append Table 6.33 giving a summary
of the results in all the cases hitherto ascertained. The fertility of the
unions may be judged by two standards, namely, by the proportion of flowers
which, when fertilised in the two methods, yield capsules, and by the average
number of seeds per capsule. When there is a dash in the left hand column
opposite to the name of the species, the proportion of the flowers which yielded
capsules was not recorded.
TABLE 6.33. Fertility of the legitimate unions taken together, compared with
that of the illegitimate unions together. The fertility of the legitimate
unions, as judged by both standards, is taken as 100.
Column 1: Name of species.
Column 2: Illegitimate unions : proportional number of flowers which produced
capsules.
Column 3: Illegitimate unions : average number of seeds per capsule.
Primula veris : 69 : 65.
Primula elatior : 27 : 75.
Primula vulgaris : 60 : 54.
Primula Sinensis : 84 : 63.
Primula Sinensis (second trial) : 0 : 53.
Primula Sinensis (Hildebrand) : 100 : 42.
Primula auricula (Scott) : 80 : 15.
Primula Sikkimensis (Scott) : 95 : 31.
Primula cortusoides (Scott) : 74 : 66.
Primula involucrata (Scott) : 72 : 48.
Primula farinosa (Scott) : 71 : 44.
Average of the nine species of Primula : 88.4 : 69.
Hottonia palustris (H. Muller) : - : 61.
Linum grandiflorum (the difference probably is much greater) : - : 69.
Linum perenne : - : 20.
Linum perenne (Hildebrand) : 0 : 0.
Pulmonaria officinalis (German stock, Hildebrand) : 0 : 0.
Pulmonaria angustifolia : 35 : 32.
Mitchella repens : 20 : 47.
Borreria, Brazilian sp. : - : 0.
Polygonum fagopyrum : - : 46.
Lythrum salicaria : 33 : 46.
Oxalis Valdiviana (Hildebrand) : 2 : 34.
Oxalis Regnelli : 0 : 0.
Oxalis speciosa : 15 : 49.
The two or three forms of the same heterostyled species do not differ from one
another in general habit or foliage, as sometimes, though rarely, happens with
the two sexes of dioecious plants. Nor does the calyx differ, but the corolla
sometimes differs slightly in shape, owing to the different position of the
anthers. In Borreria the hairs within the tube of the corolla are differently
situated in the long-styled and short-styled forms. In Pulmonaria there is a
slight difference in the size of the corolla, and in Pontederia in its colour.
In the reproductive organs the differences are much greater and more important.
In the one form the stamens may be all of the same length, and in the other
graduated in length, or alternately longer and shorter. The filaments may differ
in colour and thickness, and are sometimes nearly thrice as long in the one form
as in the other. They adhere also for very different proportional lengths to the
corolla. The anthers sometimes differ much in size in the two forms. Owing to
the rotation of the filaments, the anthers, when mature, dehisce towards the
circumference of the flower in one form of Faramea, and towards the centre in
the other form. The pollen-grains sometimes differ conspicuously in colour, and
often to an extraordinary degree in diameter. They differ also somewhat in
shape, and apparently in their contents, as they are unequally opaque. In the
short-styled form of Faramea the pollen-grains are covered with sharp points, so
as to cohere readily together or to an insect; whilst the smaller grains of the
long-styled form are quite smooth.
With respect to the pistil, the style may be almost thrice as long in the one
form as in the other. In Oxalis it sometimes differs in hairiness in the three
forms. In Linum the pistils either diverge and pass out between the filaments,
or stand nearly upright and parallel to them. The stigmas in the two forms often
differ much in size and shape, and more especially in the length and thickness
of their papillae; so that the surface may be rough or quite smooth. Owing to
the rotation of the styles, the papillose surface of the stigma is turned
outwards in one form of Linum perenne, and inwards in the other form. In flowers
of the same age of Primula veris the ovules are larger in the long-styled than
in the short-styled form. The seeds produced by the two or three forms often
differ in number, and sometimes in size and weight; thus, five seeds from the
long-styled form of Lythrum salicaria equal in weight six from the mid-styled
and seven from the short-styled form. Lastly, short-styled plants of Pulmonaria
officinalis bear a larger number of flowers, and these set a larger proportional
number of fruit, which however yield a lower average number of seed, than the
long-styled plants. With heterostyled plants we thus see in how many and in what
important characters the forms of the same undoubted species often differ from
one another--characters which with ordinary plants would be amply sufficient to
distinguish species of the same genus.
As the pollen-grains of ordinary species belonging to the same genus generally
resemble one another closely in all respects, it is worth while to show, in
Table 6.34, the difference in diameter between the grains from the two or three
forms of the same heterostyled species in the forty-three cases in which this
was ascertained. But it should be observed that some of the following
measurements are only approximately accurate, as only a few grains were
measured. In several cases, also, the grains had been dried and were then soaked
in water. Whenever they were of an elongated shape their longer diameters were
measured. The grains from the short-styled plants are invariably larger than
those from the long-styled, whenever there is any difference between them. The
diameter of the former is represented in the table by the number 100.
TABLE 6.34. Relative diameter of the pollen-grains from the forms of the same
heterostyled species; those from the short-styled form being represented by 100.
DIMORPHIC SPECIES.
Column 1: Name of species.
Column 2: From the long-styled form : relative diameter.
Primula veris : 67.
Primula vulgaris : 71.
Primula Sinensis (Hildebrand) : 57.
Primula auricula : 71.
Hottonia palustris (H. Muller) : 61.
Hottonia palustris (self) : 64.
Linum grandiflorum : 100.
Linum perenne (diameter variable) : 100 (?).
Linum flavum : 100.
Pulmonaria officinalis : 78.
Pulmonaria angustifolia : 91.
Polygonum fagopyrum : 82.
Leucosmia Burnettiana : 99.
Aegiphila elata : 62.
Menyanthes trifoliata : 84.
Limnanthemum Indicum : 100.
Villarsia (sp.?) : 75.
Forsythia suspensa : 94.
Cordia (sp.?) : 100.
Gilia pulchella : 100.
Gilia micrantha : 81.
Sethia acuminata : 83.
Erythroxylum (sp.?) : 93.
Cratoxylon formosum : 86.
Mitchella repens, pollen-grains of the long-styled a little smaller.
Borreria (sp.?) : 92.
Faramea (sp.?) : 67.
Suteria (sp.?) (Fritz Muller) : 75.
Houstonia coerulea : 72.
Oldenlandia (sp.?) : 78.
Hedyotis (sp.?) : 88.
Coccocypselum (sp.?) (Fritz Muller) : 100.
Lipostoma (sp.?) : 80.
Cinchona micrantha : 91.
TRIMORPHIC SPECIES.
Column 1: Name of species.
Column 2: Ratio expressing the extreme differences in diameter of the pollen-
grains from the two sets of anthers in the three forms.
Lythrum salicaria : 60.
Nesaea verticillata : 65.
Oxalis Valdiviana (Hildebrand) : 71.
Oxalis Regnelli : 78.
Oxalis speciosa : 69.
Oxalis sensitiva : 84.
Pontederia (sp.?) : 55.
Column 1: Name of species.
Column 2: Ratio between the diameters of the pollen-grains of the two sets of
anthers in the same form.
Oxalis rosea, long-styled form (Hildebrand) : 83.
Oxalis compressa, short-styled form : 83.
Pontederia (sp.?) short-styled form : 87.
Pontederia other sp. mid-styled form : 86.
We here see that, with seven or eight exceptions out of the forty-three cases,
the pollen-grains from one form are larger than those from the other form of the
same species. The extreme difference is as 100 to 55; and we should bear in mind
that in the case of spheres differing to this degree in diameter, their contents
differ in the ratio of six to one. With all the species in which the grains
differ in diameter, there is no exception to the rule that those from the
anthers of the short-styled form, the tubes of which have to penetrate the
longer pistil of the long-styled form, are larger than the grains from the other
form. This curious relation led Delpino (as it formerly did me) to believe that
the larger size of the grains in the short-styled flowers is connected with the
greater supply of matter needed for the development of their longer tubes. (6/2.
'Sull' Opera, la Distribuzione dei Sessi nelle Piante' etc 1867 page 17.) But
the case of Linum, in which the grains of the two forms are of equal size,
whilst the pistil of the one is about twice as long as that of the other, made
me from the first feel very doubtful with respect to this view. My doubts have
since been strengthened by the cases of Limnanthemum and Coccocypselum, in which
the grains are of equal size in the two forms; whilst in the former genus the
pistil is nearly thrice and in the latter twice as long as in the other form. In
those species in which the grains are of unequal size in the two forms, there is
no close relationship between the degree of their inequality and that of their
pistils. Thus in Pulmonaria officinalis and in Erythroxylum the pistil in the
long-styled form is about twice the length of that in the other form, whilst in
the former species the pollen-grains are as 100 to 78, and in the latter as 100
to 93 in diameter. In the two forms of Suteria the pistil differs but little in
length, whilst the pollen-grains are as 100 to 75 in diameter. These cases seem
to prove that the difference in size between the grains in the two forms is not
determined by the length of the pistil, down which the tubes have to grow. That
with plants in general there is no close relationship between the size of the
pollen-grains and the length of the pistil is manifest: for instance, I found
that the distended grains of Datura arborea were .00243 of an inch in diameter,
and the pistil no less than 9.25 inches in length; now the pistil in the small
flowers of Polygonum fagopyrum is very short, yet the larger pollen-grains from
the short-styled plants had exactly the same diameter as those from the Datura,
with its enormously elongated pistil.
Notwithstanding these several considerations, it is difficult quite to give up
the belief that the pollen-grains from the longer stamens of heterostyled plants
have become larger in order to allow of the development of longer tubes; and the
foregoing opposing facts may possibly be reconciled in the following manner. The
tubes are at first developed from matter contained within the grains, for they
are sometimes exserted to a considerable length, before the grains have touched
the stigma; but botanists believe that they afterwards draw nourishment from the
conducting tissue of the pistil. It is hardly possible to doubt that this must
occur in such cases as that of the Datura, in which the tubes have to grow down
the whole length of the pistil, and therefore to a length equalling 3,806 times
the diameter of the grains (namely, .00243 of an inch) from which they are
protruded. I may here remark that I have seen the pollen-grains of a willow,
immersed in a very weak solution of honey, protrude their tubes, in the course
of twelve hours, to a length thirteen times as great as the diameter of the
grains. Now if we suppose that the tubes in some heterostyled species are
developed wholly or almost wholly from matter contained within the grains, while
in other species from matter yielded by the pistil, we can see that in the
former case it would be necessary that the grains of the two forms should differ
in size relatively to the length of the pistil which the tubes have to
penetrate, but that in the latter case it would not be necessary that the grains
should thus differ. Whether this explanation can be considered satisfactory must
remain at present doubtful.
There is another remarkable difference between the forms of several heterostyled
species, namely in the anthers of the short-styled flowers, which contain the
larger pollen-grains, being longer than those of the long-styled flowers. This
is the case with Hottonia palustris in the ratio of 100 to 83. With Limnanthemum
Indicum the ratio is as 100 to 70. With the allied Menyanthes the anthers of the
short-styled form are a little and with Villarsia conspicuously larger than
those of the long-styled. With Pulmonaria angustifolia they vary much in size,
but from an average of seven measurements of each kind the ratio is as 100 to
91. In six genera of the Rubiaceae there is a similar difference, either
slightly or well marked. Lastly, in the trimorphic Pontederia the ratio is 100
to 88; the anthers from the longest stamens in the short-styled form being
compared with those from the shortest stamens in the long-styled form. On the
other hand, there is a similar and well-marked difference in the length of the
stamens in the two forms of Forsythia suspensa and of Linum flavum; but in these
two cases the anthers of the short-styled flowers are shorter than those of the
long-styled. The relative size of the anthers was not particularly attended to
in the two forms of the other heterostyled plants, but I believe that they are
generally equal, as is certainly the case with those of the common primrose and
cowslip.
The pistil differs in length in the two forms of every heterostyled plant, and
although a similar difference is very general with the stamens, yet in the two
forms of Linum grandiflorum and of Cordia they are equal. There can hardly be a
doubt that the relative length of these organs is an adaptation for the safe
transportal by insects of the pollen from the one form to the other. The
exceptional cases in which these organs do not stand exactly on a level in the
two forms may probably be explained by the manner in which the flowers are
visited. With most of the species, if there is any difference in the size of the
stigma of the two forms, that of the long-styled, whatever its shape may be, is
larger than that of the short-styled. But here again there are some exceptions
to the rule, for in the short-styled form of Leucosmia Burnettiana the stigmas
are longer and much narrower than those of the long-styled; the ratio between
the lengths of the stigmas in the two forms being 100 to 60. In the three
Rubiaceous genera, Faramea, Houstonia and Oldenlandia, the stigmas of the short-
styled form are likewise somewhat longer and narrower; and in the three forms of
Oxalis sensitiva the difference is strongly marked, for if the length of the two
stigmas of the long-styled pistil be taken as 100, it will be represented in the
mid- and short-styled forms by the numbers 141 and 164. As in all these cases
the stigmas of the short-styled pistil are seated low down within a more or less
tubular corolla, it is probable that they are better fitted by being long and
narrow for brushing the pollen off the inserted proboscis of an insect.
With many heterostyled plants the stigma differs in roughness in the two forms,
and when this is the case there is no known exception to the rule that the
papillae on the stigma of the long-styled form are longer and often thicker than
those on that of the short-styled. For instance, the papillae on the long-styled
stigma of Hottonia palustris are more than twice the length of those in the
other form. This holds good even in the case of Houstonia coerulea, in which the
stigmas are much shorter and stouter in the long-styled than in the short-styled
form, for the papillae on the former compared with those on the latter are as
100 to 58 in length. The length of the pistil in the long-styled form of Linum
grandiflorum varies much, and the stigmatic papillae vary in a corresponding
manner. From this fact I inferred at first that in all cases the difference in
length between the stigmatic papillae in the two forms was one merely of
correlated growth; but this can hardly be the true or general explanation, as
the shorter stigmas of the long-styled form of Houstonia have the longer
papillae. It is a more probable view that the papillae, which render the stigma
of the long-styled form of various species rough, serve to entangle effectually
the large-sized pollen-grains brought by insects from the short-styled form,
thus ensuring its legitimate fertilisation. This view is supported by the fact
that the pollen-grains from the two forms of eight species in Table 6.34 hardly
differ in diameter, and the papillae on their stigmas do not differ in length.
The species which are at present positively or almost positively known to be
heterostyled belong, as shown in Table 6.35, to 38 genera, widely distributed
throughout the world. These genera are included in fourteen Families, most of
which are very distinct from one another, for they belong to nine of the several
great Series, into which phanerogamic plants have been divided by Bentham and
Hooker.
TABLE 6.35. List of genera including heterostyled species.
DICOTYLEDONS.
HYPERICINEAE:
Cratoxylon.
ERYTHROXYLEAE:
Erythroxylum.
Sethia.
GERANIACEAE:
Linum.
Oxalis.
LYTHRACEAE:
Lythrum.
Neseae.
RUBIACEAE:
Cinchona.
Bouvardia.
Manettia.
Hedyotis.
Oldenlandia.
Houstonia.
Coccocypselum.
Lipostoma.
Knoxia.
Faramea.
Psychotria.
Rudgea.
Suteria.
Mitchella.
Diodia.
Borreria.
Spermacoce.
PRIMULACEAE:
Primula.
Hottonia.
Androsace.
OLEACEAE:
Forsythia.
GENTIANACEAE:
Menyanthes.
Limnanthemum.
Villarsia.
POLEMONIACEAE:
Gilia.
CORDIEAE:
Cordia.
BORAGINEAE:
Pulmonaria.
VERBENACEAE:
Aegiphila.
POLYGONEAE:
Polygonum.
THYMELEAE:
Thymelea.
MONOCOTYLEDONS.
PONTEDERIACEAE:
Pontederia.
In some of these families the heterostyled condition must have been acquired at
a very remote period. Thus the three closely allied genera, Menyanthes,
Limnanthemum, and Villarsia, inhabit respectively Europe, India, and South
America. Heterostyled species of Hedyotis are found in the temperate regions of
North and the tropical regions of South America. Trimorphic species of Oxalis
live on both sides of the Cordillera in South America and at the Cape of Good
Hope. In these and some other cases it is not probable that each species
acquired its heterostyled structure independently of its close allies. If they
did not do so, the three closely connected genera of the Menyantheae and the
several trimorphic species of Oxalis must have inherited their structure from a
common progenitor. But an immense lapse of time will have been necessary in all
such cases for the modified descendants of a common progenitor to have spread
from a single centre to such widely remote and separated areas. The family of
the Rubiaceae contains not far short of as many heterostyled genera as all the
other thirteen families together; and hereafter no doubt other Rubiaceous genera
will be found to be heterostyled, although a large majority are homostyled.
Several closely allied genera in this family probably owe their heterostyled
structure to descent in common; but as the genera thus characterised are
distributed in no less than eight of the tribes into which this family has been
divided by Bentham and Hooker, it is almost certain that several of them must
have become heterostyled independently of one another. What there is in the
constitution or structure of the members of this family which favours their
becoming heterostyled, I cannot conjecture. Some families of considerable size,
such as the Boragineae and Verbenaceae, include, as far as is at present known,
only a single heterostyled genus. Polygonum also is the sole heterostyled genus
in its family; and though it is a very large genus, no other species except P.
fagopyrum is thus characterised. We may suspect that it has become heterostyled
within a comparatively recent period, as it seems to be less strongly so in
function than the species in any other genus, for both forms are capable of
yielding a considerable number of spontaneously self-fertilised seeds. Polygonum
in possessing only a single heterostyled species is an extreme case; but every
other genus of considerable size which includes some such species likewise
contains homostyled species. Lythrum includes trimorphic, dimorphic, and
homostyled species.
Trees, bushes, and herbaceous plants, both large and small, bearing single
flowers or flowers in dense spikes or heads, have been rendered heterostyled. So
have plants which inhabit alpine and lowland sites, dry land, marshes and water.
(6/3. Out of the 38 genera known to include heterostyled species, about eight,
or 21 per cent, are more or less aquatic in their habits. I was at first struck
with this fact, for I was not then aware how large a proportion of ordinary
plants inhabit such stations. Heterostyled plants may be said in one sense to
have their sexes separated, as the forms must mutually fertilise one another.
Therefore it seemed worth while to ascertain what proportion of the genera in
the Linnean classes, Monoecia, Dioecia and Polygamia, contained species which
live "in water, marshes, bogs or watery places." In Sir W.J. Hooker's 'British
Flora' 4th edition 1838, these three Linnean classes include 40 genera, 17 of
which (i.e. 43 per cent) contain species inhabiting the just-specified stations.
So that 43 per cent of those British plants which have their sexes separated are
more or less aquatic in their habits, whereas only 21 per cent of heterostyled
plants have such habits. I may add that the hermaphrodite classes, from
Monandria to Gynandria inclusive, contain 447 genera, of which 113 are aquatic
in the above sense, or only 25 per cent. It thus appears, as far as can be
judged from such imperfect data, that there is some connection between the
separation of the sexes in plants and the watery nature of the sites which they
inhabit; but that this does not hold good with heterostyled species.)
When I first began to experimentise on heterostyled plants it was under the
impression that they were tending to become dioecious; but I was soon forced to
relinquish this notion, as the long-styled plants of Primula which, from
possessing a longer pistil, larger stigma, shorter stamens with smaller pollen-
grains, seemed to be the more feminine of the two forms, yielded fewer seeds
than the short-styled plants which appeared to be in the above respects the more
masculine of the two. Moreover, trimorphic plants evidently come under the same
category with dimorphic, and the former cannot be looked at as tending to become
dioecious. With Lythrum salicaria, however, we have the curious and unique case
of the mid-styled form being more feminine or less masculine in nature than the
other two forms. This is shown by the large number of seeds which it yields in
whatever manner it may be fertilised, and by its pollen (the grains of which are
of smaller size than those from the corresponding stamens in the other two
forms) when applied to the stigma of any form producing fewer seeds than the
normal number. If we suppose the process of deterioration of the male organs in
the mid-styled form to continue, the final result would be the production of a
female plant; and Lythrum salicaria would then consist of two heterostyled
hermaphrodites and a female. No such case is known to exist, but it is a
possible one, as hermaphrodite and female forms of the same species are by no
means rare. Although there is no reason to believe that heterostyled plants are
regularly becoming dioecious, yet they offer singular facilities, as will
hereafter be shown, for such conversion; and this appears occasionally to have
been effected.
We may feel sure that plants have been rendered heterostyled to ensure cross-
fertilisation, for we now know that a cross between the distinct individuals of
the same species is highly important for the vigour and fertility of the
offspring. The same end is gained by dichogamy or the maturation of the
reproductive elements of the same flower at different periods,--by
dioeciousness--self-sterility--the prepotency of pollen from another individual
over a plant's own pollen,--and lastly, by the structure of the flower in
relation to the visits of insects. The wonderful diversity of the means for
gaining the same end in this case, and in many others, depends on the nature of
all the previous changes through which the species has passed, and on the more
or less complete inheritance of the successive adaptations of each part to the
surrounding conditions. Plants which are already well adapted by the structure
of their flowers for cross-fertilisation by the aid of insects often possess an
irregular corolla, which has been modelled in relation to their visits; and it
would have been of little or no use to such plants to have become heterostyled.
We can thus understand why it is that not a single species is heterostyled in
such great families as the Leguminosae, Labiatae, Scrophulariaceae, Orchideae,
etc., all of which have irregular flowers. Every known heterostyled plant,
however, depends on insects for its fertilisation, and not on the wind; so that
it is a rather surprising fact that only one genus, Pontederia, has a plainly
irregular corolla.
Why some species are adapted for cross-fertilisation, whilst others within the
same genus are not so, or if they once were, have since lost such adaptation and
in consequence are now usually self-fertilised, I have endeavoured elsewhere to
explain to a certain limited extent. (6/4. 'The Effects of Cross and Self-
fertilisation' 1876 page 441.) If it be further asked why some species have been
adapted for this end by being made heterostyled, rather than by any of the above
specified means, the answer probably lies in the manner in which heterostylism
originated,--a subject immediately to be discussed. Heterostyled species,
however, have an advantage over dichogamous species, as all the flowers on the
same heterostyled plant belong to the same form, so that when fertilised
legitimately by insects two distinct individuals are sure to intercross. On the
other hand, with dichogamous plants, early or late flowers on the same
individual may intercross; and a cross of this kind does hardly any or no good.
Whenever it is profitable to a species to produce a large number of seeds and
this obviously is a very common case, heterostyled will have an advantage over
dioecious plants, as all the individuals of the former, whilst only half of the
latter, that is the females, yield seeds. On the other hand, heterostyled plants
seem to have no advantage, as far as cross-fertilisation is concerned, over
those which are sterile with their own pollen. They lie indeed under a slight
disadvantage, for if two self-sterile plants grow near together and far removed
from all other plants of the same species, they will mutually and perfectly
fertilise one another, whilst this will not be the case with heterostyled
dimorphic plants, unless they chance to belong to opposite forms.
It may be added that species which are trimorphic have one slight advantage over
the dimorphic; for if only two individuals of a dimorphic species happen to grow
near together in an isolated spot, the chances are even that both will belong to
the same form, and in this case they will not produce the full number of
vigorous and fertile seedlings; all these, moreover, will tend strongly to
belong to the same form as their parents. On the other hand, if two plants of
the same trimorphic species happen to grow in an isolated spot, the chances are
two to one in favour of their not belonging to the same form; and in this case
they will legitimately fertilise one another, and yield the full complement of
vigorous offspring.
THE MEANS BY WHICH PLANTS MAY HAVE BEEN RENDERED HETEROSTYLED.
This is a very obscure subject, on which I can throw little light, but which is
worthy of discussion. It has been shown that heterostyled plants occur in
fourteen natural families, dispersed throughout the whole vegetable kingdom, and
that even within the family of the Rubiaceae they are dispersed in eight of the
tribes. We may therefore conclude that this structure has been acquired by
various plants independently of inheritance from a common progenitor, and that
it can be acquired without any great difficulty--that is, without any very
unusual combination of circumstances.
It is probable that the first step towards a species becoming heterostyled is
great variability in the length of the pistil and stamens, or of the pistil
alone. Such variations are not very rare: with Amsinckia spectabilis and Nolana
prostrata these organs differ so much in length in different individuals that,
until experimenting on them, I thought both species heterostyled. The stigma of
Gesneria pendulina sometimes protrudes far beyond, and is sometimes seated
beneath the anthers; so it is with Oxalis acetosella and various other plants. I
have also noticed an extraordinary amount of difference in the length of the
pistil in cultivated varieties of Primula veris and vulgaris.
As most plants are at least occasionally cross-fertilised by the aid of insects,
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