The Effects of Cross & Self-Fertilisation in the Vegetable Kingdom
by
Charles Darwin
Part 2 out of 10
were fertilised with pollen from the same flower; and from the seeds
thus produced, self-fertilised plants of the eighth generation
(grandchildren of Hero) were raised. Several other flowers on the same
plants were crossed with pollen from the other children of Hero. The
seedlings raised from this cross may be considered as the offspring of
the union of brothers and sisters. The result of the competition between
these two sets of seedlings (namely self-fertilised and the offspring of
brothers and sisters) is given in Table 2/16.
TABLE 2/16. Ipomoea purpurea.
Heights of Plants in inches:
Column 1: Number (Name) of Pot.
Column 2: Self-fertilised Grandchildren of Hero, from the
Self-fertilised Children. Eighth Generation.
Column 3: Grandchildren from a cross between the self-fertilised
children of Hero. Eighth Generation.
Pot 1 : 86 6/8 : 95 6/8.
Pot 1 : 90 3/8 : 95 3/8.
Pot 2 : 96 : 85.
Pot 2 : 77 2/8 : 93.
Pot 3 : 73 : 86 2/8.
Pot 3 : 66 : 82 2/8.
Pot 3 : 84 4/8 : 70 6/8.
Pot 4 : 88 1/8 : 66 3/8.
Pot 4 : 84 : 15 4/8.
Pot 4 : 36 2/8 : 38.
Pot 4 : 74 : 78 3/8.
Pot 5 : 90 1/8 : 82 6/8.
Pot 5 : 90 5/8 : 83 6/8.
Total : 1037.00 : 973.16.
The average height of the thirteen self-fertilised grandchildren of Hero
is 79.76 inches, and that of the grandchildren from a cross between the
self-fertilised children is 74.85; or as 100 to 94. But in Pot 4 one of
the crossed plants grew only to a height of 15 1/2 inches; and if this
plant and its opponent are struck out, as would be the fairest plan, the
average height of the crossed plants exceeds only by a fraction of an
inch that of the self-fertilised plants. It is therefore clear that a
cross between the self-fertilised children of Hero did not produce any
beneficial effect worth notice; and it is very doubtful whether this
negative result can be attributed merely to the fact of brothers and
sisters having been united, for the ordinary intercrossed plants of the
several successive generations must often have been derived from the
union of brothers and sisters (as shown in Chapter 1), and yet all of
them were greatly superior to the self-fertilised plants. We are
therefore driven to the suspicion, which we shall soon see strengthened,
that Hero transmitted to its offspring a peculiar constitution adapted
for self-fertilisation.
It would appear that the self-fertilised descendants of Hero have not
only inherited from Hero a power of growth equal to that of the ordinary
intercrossed plants, but have become more fertile when self-fertilised
than is usual with the plants of the present species. The flowers on the
self-fertilised grandchildren of Hero in Table 2.16 (the eighth
generation of self-fertilised plants) were fertilised with their own
pollen and produced plenty of capsules, ten of which (though this is too
few a number for a safe average) contained 5.2 seeds per capsule,--a
higher average than was observed in any other case with the
self-fertilised plants. The anthers produced by these self-fertilised
grandchildren were also as well developed and contained as much pollen
as those on the intercrossed plants of the corresponding generation;
whereas this was not the case with the ordinary self-fertilised plants
of the later generations. Nevertheless some few of the flowers produced
by the grandchildren of Hero were slightly monstrous, like those of the
ordinary self-fertilised plants of the later generations. In order not
to recur to the subject of fertility, I may add that twenty-one
self-fertilised capsules, spontaneously produced by the
great-grandchildren of Hero (forming the ninth generation of
self-fertilised plants), contained on an average 4.47 seeds; and this is
as high an average as the self-fertilised flowers of any generation
usually yielded.
Several flowers on the self-fertilised grandchildren of Hero in Table
2/16 were fertilised with pollen from the same flower; and the seedlings
raised from them (great-grandchildren of Hero) formed the ninth
self-fertilised generation. Several other flowers were crossed with
pollen from another grandchild, so that they may be considered as the
offspring of brothers and sisters, and the seedlings thus raised may be
called the INTERCROSSED great-grandchildren. And lastly, other flowers
were fertilised with pollen from a distinct stock, and the seedlings
thus raised may be called the COLCHESTER-CROSSED great-grandchildren. In
my anxiety to see what the result would be, I unfortunately planted the
three lots of seeds (after they had germinated on sand) in the hothouse
in the middle of winter, and in consequence of this the seedlings
(twenty in number of each kind) became very unhealthy, some growing only
a few inches in height, and very few to their proper height. The result,
therefore, cannot be fully trusted; and it would be useless to give the
measurements in detail. In order to strike as fair an average as
possible, I first excluded all the plants under 50 inches in height,
thus rejecting all the most unhealthy plants. The six self-fertilised
thus left were on an average 66.86 inches high; the eight intercrossed
plants 63.2 high; and the seven Colchester-crossed 65.37 high; so that
there was not much difference between the three sets, the
self-fertilised plants having a slight advantage. Nor was there any
great difference when only the plants under 36 inches in height were
excluded. Nor again when all the plants, however much dwarfed and
unhealthy, were included. In this latter case the Colchester-crossed
gave the lowest average of all; and if these plants had been in any
marked manner superior to the other two lots, as from my former
experience I fully expected they would have been, I cannot but think
that some vestige of such superiority would have been evident,
notwithstanding the very unhealthy condition of most of the plants. No
advantage, as far as we can judge, was derived from intercrossing two of
the grandchildren of Hero, any more than when two of the children were
crossed. It appears therefore that Hero and its descendants have varied
from the common type, not only in acquiring great power of growth, and
increased fertility when subjected to self-fertilisation, but in not
profiting from a cross with a distinct stock; and this latter fact, if
trustworthy, is a unique case, as far as I have observed in all my
experiments.]
SUMMARY ON THE GROWTH, VIGOUR, AND FERTILITY OF THE SUCCESSIVE
GENERATIONS OF THE CROSSED AND SELF-FERTILISED PLANTS OF Ipomoea
purpurea, TOGETHER WITH SOME MISCELLANEOUS OBSERVATIONS.
In Table 2/17, we see the average or mean heights of the ten successive
generations of the intercrossed and self-fertilised plants, grown in
competition with each other; and in the right hand column we have the
ratios of the one to the other, the height of the intercrossed plants
being taken at 100. In the bottom line the mean height of the
seventy-three intercrossed plants is shown to be 85.84 inches, and that
of the seventy-three self-fertilised plants 66.02 inches, or as 100 to
77.
TABLE 2/17. Ipomoea purpurea. Summary of measurements of the ten
generations.
Heights of Plants in inches:
Column 1: Name of Generation.
Column 2: Number of Crossed Plants.
Column 3: Average Height of Crossed Plants.
Column 4: Number of Self-fertilised Plants.
Column 5: Average Height of Self-fertilised Plants.
Column 6: n in Ratio between Average Heights of Crossed and
Self-fertilised Plants, expressed as 100 to n.
First generation Table 2/1 : 6 : 86.00 : 6 : 65.66 : 76.
Second generation Table 2/2 : 6 : 84.16 : 6 : 66.33 : 79.
Third generation Table 2/3 : 6 : 77.41 : 6 : 52.83 : 68.
Fourth generation Table 2/5 : 7 : 69.78 : 7 : 60.14 : 86.
Fifth generation Table 2/6 : 6 : 82.54 : 6 : 62.33 : 75.
Sixth generation Table 2/7 : 6 : 87.50 : 6 : 63.16 : 72.
Seventh generation Table 2/8 : 9 : 83.94 : 9 : 68.25 : 81.
Eighth generation Table 2/9 : 8 : 113.25 : 8 : 96.65 : 85.
Ninth generation Table 2/10 : 14 : 81.39 : 14 : 64.07 : 79.
Tenth generation Table 2/11 : 5 : 93.70 : 5 : 50.40 : 54.
All ten generations together : 73 : 85.84 : 73 : 66.02 : 77.
(DIAGRAM 2/1. Diagram showing the mean heights of the crossed and
self-fertilised plants of Ipomoea purpurea in the ten generations; the
mean height of the crossed plants being taken as 100. On the right hand,
the mean heights of the crossed and self-fertilised plants of all the
generations taken together are shown (as eleven pairs of unequal
vertical lines.))
The mean height of the self-fertilised plants in each of the ten
generations is also shown in the diagram 2/1, that of the intercrossed
plants being taken at 100, and on the right side we see the relative
heights of the seventy-three intercrossed plants, and of the
seventy-three self-fertilised plants. The difference in height between
the crossed and self-fertilised plants will perhaps be best appreciated
by an illustration: If all the men in a country were on an average 6
feet high, and there were some families which had been long and closely
interbred, these would be almost dwarfs, their average height during ten
generations being only 4 feet 8 1/4 inches.
It should be especially observed that the average difference between the
crossed and self-fertilised plants is not due to a few of the former
having grown to an extraordinary height, or to a few of the
self-fertilised being extremely short, but to all the crossed plants
having surpassed their self-fertilised opponents, with the few following
exceptions. The first occurred in the sixth generation, in which the
plant named "Hero" appeared; two in the eighth generation, but the
self-fertilised plants in this generation were in an anomalous
condition, as they grew at first at an unusual rate and conquered for a
time the opposed crossed plants; and two exceptions in the ninth
generation, though one of these plants only equalled its crossed
opponent. Therefore, of the seventy-three crossed plants, sixty-eight
grew to a greater height than the self-fertilised plants, to which they
were opposed.
In the right-hand column of figures, the difference in height between
the crossed and self-fertilised plants in the successive generations is
seen to fluctuate much, as might indeed have been expected from the
small number of plants measured in each generation being insufficient to
give a fair average. It should be remembered that the absolute height of
the plants goes for nothing, as each pair was measured as soon as one of
them had twined up to the summit of its rod. The great difference in the
tenth generation, namely, 100 to 54, no doubt was partly accidental,
though, when these plants were weighed, the difference was even greater,
namely, 100 to 44. The smallest amount of difference occurred in the
fourth and the eighth generations, and this was apparently due to both
the crossed and self-fertilised plants having become unhealthy, which
prevented the former attaining their usual degree of superiority. This
was an unfortunate circumstance, but my experiments were not thus
vitiated, as both lots of plants were exposed to the same conditions,
whether favourable or unfavourable.
There is reason to believe that the flowers of this Ipomoea, when
growing out of doors, are habitually crossed by insects, so that the
first seedlings which I raised from purchased seeds were probably the
offspring of a cross. I infer that this is the case, firstly from
humble-bees often visiting the flowers, and from the quantity of pollen
left by them on the stigmas of such flowers; and, secondly, from the
plants raised from the same lot of seed varying greatly in the colour of
their flowers, for as we shall hereafter see, this indicates much
intercrossing. (2/3. Verlot says 'Sur la Production des Variétés' 1865
page 66, that certain varieties of a closely allied plant, the
Convolvulus tricolor, cannot be kept pure unless grown at a distance
from all other varieties.) It is, therefore, remarkable that the plants
raised by me from flowers which were, in all probability,
self-fertilised for the first time after many generations of crossing,
should have been so markedly inferior in height to the intercrossed
plants as they were, namely, as 76 to 100. As the plants which were
self-fertilised in each succeeding generation necessarily became much
more closely interbred in the later than in the earlier generations, it
might have been expected that the difference in height between them and
the crossed plants would have gone on increasing; but, so far is this
from being the case, that the difference between the two sets of plants
in the seventh, eighth, and ninth generations taken together is less
than in the first and second generations together. When, however, we
remember that the self-fertilised and crossed plants are all descended
from the same mother-plant, that many of the crossed plants in each
generation were related, often closely related, and that all were
exposed to the same conditions, which, as we shall hereafter find, is a
very important circumstance, it is not at all surprising that the
difference between them should have somewhat decreased in the later
generations. It is, on the contrary, an astonishing fact, that the
crossed plants should have been victorious, even to a slight degree,
over the self-fertilised plants of the later generations.
The much greater constitutional vigour of the crossed than of the
self-fertilised plants, was proved on five occasions in various ways;
namely, by exposing them, while young, to a low temperature or to a
sudden change of temperature, or by growing them, under very
unfavourable conditions, in competition with full-grown plants of other
kinds.
With respect to the productiveness of the crossed and self-fertilised
plants of the successive generations, my observations unfortunately were
not made on any uniform plan, partly from the want of time, and partly
from not having at first intended to observe more than a single
generation. A summary of the results is here given in a tabulated form,
the fertility of the crossed plants being taken as 100.
TABLE 2/18. Ratio of productiveness of crossed and self-fertilised
plants. Ipomoea purpurea.
FIRST GENERATION OF CROSSED AND SELF-FERTILISED PLANTS GROWING IN
COMPETITION WITH ONE ANOTHER.
Sixty-five capsules produced from flowers on five crossed plants
fertilised by pollen from a distinct plant, and fifty-five capsules
produced from flowers on five self-fertilised plants fertilised by their
own pollen, contained seeds in the proportion of : 100 to 93.
Fifty-six spontaneously self-fertilised capsules on the above five
crossed plants, and twenty-five spontaneously self-fertilised capsules
on the above five self-fertilised plants, yielded seeds in the
proportion of : 100 to 99.
Combining the total number of capsules produced by these plants, and the
average number of seeds in each, the above crossed and self-fertilised
plants yielded seeds in the proportion of : 100 to 64.
Other plants of this first generation grown under unfavourable
conditions and spontaneously self-fertilised, yielded seeds in the
proportion of : 100 to 45.
THIRD GENERATION OF CROSSED AND SELF-FERTILISED PLANTS.
Crossed capsules compared with self-fertilised capsules contained seeds
in the ratio of : 100 to 94.
An equal number of crossed and self-fertilised plants, both
spontaneously self-fertilised, produced capsules in the ratio of : 100
to 38.
And these capsules contained seeds in the ratio of : 100 to 94.
Combining these data, the productiveness of the crossed to the
self-fertilised plants, both spontaneously self-fertilised, was as : 100
to 35.
FOURTH GENERATION OF CROSSED AND SELF-FERTILISED PLANTS.
Capsules from flowers on the crossed plants fertilised by pollen from
another plant, and capsules from flowers on the self-fertilised plants
fertilised with their own pollen, contained seeds in the proportion of :
100 to 94.
FIFTH GENERATION OF CROSSED AND SELF-FERTILISED PLANTS.
The crossed plants produced spontaneously a vast number more pods (not
actually counted) than the self-fertilised, and these contained seeds in
the proportion of : 100 to 89.
NINTH GENERATION OF CROSSED AND SELF-FERTILISED PLANTS.
Fourteen crossed plants, spontaneously self-fertilised, and fourteen
self-fertilised plants spontaneously self-fertilised, yielded capsules
(the average number of seeds per capsule not having been ascertained) in
the proportion of : 100 to 26.
PLANTS DERIVED FROM A CROSSED WITH A FRESH STOCK COMPARED WITH
INTERCROSSED PLANTS.
The offspring of intercrossed plants of the ninth generation, crossed by
a fresh stock, compared with plants of the same stock intercrossed
during ten generations, both sets of plants left uncovered and naturally
fertilised, produced capsules by weight as : 100 to 51.
We see in this table that the crossed plants are always in some degree
more productive than the self-fertilised plants, by whatever standard
they are compared. The degree differs greatly; but this depends chiefly
on whether an average was taken of the seeds alone, or of the capsules
alone, or of both combined. The relative superiority of the crossed
plants is chiefly due to their producing a much greater number of
capsules, and not to each capsule containing a larger average number of
seeds. For instance, in the third generation the crossed and
self-fertilised plants produced capsules in the ratio of 100 to 38,
whilst the seeds in the capsules on the crossed plants were to those on
the self-fertilised plants only as 100 to 94. In the eighth generation
the capsules on two self-fertilised plants (not included in table 2/18),
grown in separate pots and thus not subjected to any competition,
yielded the large average of 5.1 seeds. The smaller number of capsules
produced by the self-fertilised plants may be in part, but not
altogether, attributed to their lessened size or height; this being
chiefly due to their lessened constitutional vigour, so that they were
not able to compete with the crossed plants growing in the same pots.
The seeds produced by the crossed flowers on the crossed plants were not
always heavier than the self-fertilised seeds on the self-fertilised
plants. The lighter seeds, whether produced from crossed or
self-fertilised flowers, generally germinated before the heavier seeds.
I may add that the crossed plants, with very few exceptions, flowered
before their self-fertilised opponents, as might have been expected from
their greater height and vigour.
The impaired fertility of the self-fertilised plants was shown in
another way, namely, by their anthers being smaller than those in the
flowers on the crossed plants. This was first observed in the seventh
generation, but may have occurred earlier. Several anthers from flowers
on the crossed and self-fertilised plants of the eighth generation were
compared under the microscope; and those from the former were generally
longer and plainly broader than the anthers of the self-fertilised
plants. The quantity of pollen contained in one of the latter was, as
far as could be judged by the eye, about half of that contained in one
from a crossed plant. The impaired fertility of the self-fertilised
plants of the eighth generation was also shown in another manner, which
may often be observed in hybrids--namely, by the first-formed flowers
being sterile. For instance, the fifteen first flowers on a
self-fertilised plant of one of the later generations were carefully
fertilised with their own pollen, and eight of them dropped off; at the
same time fifteen flowers on a crossed plant growing in the same pot
were self-fertilised, and only one dropped off. On two other crossed
plants of the same generation, several of the earliest flowers were
observed to fertilise themselves and to produce capsules. In the plants
of the ninth, and I believe of some previous generations, very many of
the flowers, as already stated, were slightly monstrous; and this
probably was connected with their lessened fertility.
All the self-fertilised plants of the seventh generation, and I believe
of one or two previous generations, produced flowers of exactly the same
tint, namely, of a rich dark purple. So did all the plants, without any
exception, in the three succeeding generations of self-fertilised
plants; and very many were raised on account of other experiments in
progress not here recorded. My attention was first called to this fact
by my gardener remarking that there was no occasion to label the
self-fertilised plants, as they could always be known by their colour.
The flowers were as uniform in tint as those of a wild species growing
in a state of nature; whether the same tint occurred, as is probable, in
the earlier generations, neither my gardener nor self could recollect.
The flowers on the plants which were first raised from purchased seed,
as well as during the first few generations, varied much in the depth of
the purple tint; many were more or less pink, and occasionally a white
variety appeared. The crossed plants continued to the tenth generation
to vary in the same manner as before, but to a much less degree, owing,
probably, to their having become more or less closely inter-related. We
must therefore attribute the extraordinary uniformity of colour in the
flowers on the plants of the seventh and succeeding self-fertilised
generations, to inheritance not having been interfered with by crosses
during several preceding generations, in combination with the conditions
of life having been very uniform.
A plant appeared in the sixth self-fertilised generation, named the
Hero, which exceeded by a little in height its crossed antagonist, and
which transmitted its powers of growth and increased self-fertility to
its children and grandchildren. A cross between the children of Hero did
not give to the grandchildren any advantage over the self-fertilised
grandchildren raised from the self-fertilised children. And as far as my
observations can be trusted, which were made on very unhealthy plants,
the great-grandchildren raised from intercrossing the grandchildren had
no advantage over the seedlings from the grandchildren the product of
continued self-fertilisation; and what is far more remarkable, the
great-grandchildren raised by crossing the grandchildren with a fresh
stock, had no advantage over either the intercrossed or self-fertilised
great-grandchildren. It thus appears that Hero and its descendants
differed in constitution in an extraordinary manner from ordinary plants
of the present species.
Although the plants raised during ten successive generations from
crosses between distinct yet inter-related plants almost invariably
exceeded in height, constitutional vigour, and fertility their
self-fertilised opponents, it has been proved that seedlings raised by
intercrossing flowers on the same plant are by no means superior, on the
contrary are somewhat inferior in height and weight, to seedlings raised
from flowers fertilised with their own pollen. This is a remarkable
fact, which seems to indicate that self-fertilisation is in some manner
more advantageous than crossing, unless the cross brings with it, as is
generally the case, some decided and preponderant advantage; but to this
subject I shall recur in a future chapter.
The benefits which so generally follow from a cross between two plants
apparently depend on the two differing somewhat in constitution or
character. This is shown by the seedlings from the intercrossed plants
of the ninth generation, when crossed with pollen from a fresh stock,
being as superior in height and almost as superior in fertility to the
again intercrossed plants, as these latter were to seedlings from
self-fertilised plants of the corresponding generation. We thus learn
the important fact that the mere act of crossing two distinct plants,
which are in some degree inter-related and which have been long
subjected to nearly the same conditions, does little good as compared
with that from a cross between plants belonging to different stocks or
families, and which have been subjected to somewhat different
conditions. We may attribute the good derived from the crossing of the
intercrossed plants during the ten successive generations to their still
differing somewhat in constitution or character, as was indeed proved by
their flowers still differing somewhat in colour. But the several
conclusions which may be deduced from the experiments on Ipomoea will be
more fully considered in the final chapters, after all my other
observations have been given.
CHAPTER III.
SCROPHULARIACEAE, GESNERIACEAE, LABIATAE, ETC.
Mimulus luteus; height, vigour, and fertility of the crossed and
self-fertilised plants of the first four generations.
Appearance of a new, tall, and highly self-fertile variety.
Offspring from a cross between self-fertilised plants.
Effects of a cross with a fresh stock.
Effects of crossing flowers on the same plant.
Summary on Mimulus luteus.
Digitalis purpurea, superiority of the crossed plants.
Effects of crossing flowers on the same plant.
Calceolaria.
Linaria vulgaris.
Verbascum thapsus.
Vandellia nummularifolia.
Cleistogene flowers.
Gesneria pendulina.
Salvia coccinea.
Origanum vulgare, great increase of the crossed plants by stolons.
Thunbergia alata.
In the family of the Scrophulariaceae I experimented on species in the
six following genera: Mimulus, Digitalis, Calceolaria, Linaria,
Verbascum, and Vandellia.
[3/2. SCROPHULARIACEAE.--Mimulus luteus.
The plants which I raised from purchased seed varied greatly in the
colour of their flowers, so that hardly two individuals were quite
alike; the corolla being of all shades of yellow, with the most
diversified blotches of purple, crimson, orange, and coppery brown. But
these plants differed in no other respect. (3/1. I sent several
specimens with variously coloured flowers to Kew, and Dr. Hooker informs
me that they all consisted of Mimulus luteus. The flowers with much red
have been named by horticulturists as var. Youngiana.) The flowers are
evidently well adapted for fertilisation by the agency of insects; and
in the case of a closely allied species, Mimulus rosea, I have watched
bees entering the flowers, thus getting their backs well dusted with
pollen; and when they entered another flower the pollen was licked off
their backs by the two-lipped stigma, the lips of which are irritable
and close like a forceps on the pollen-grains. If no pollen is enclosed
between the lips, these open again after a time. Mr. Kitchener has
ingeniously explained the use of these movements, namely, to prevent the
self-fertilisation of the flower. (3/2. 'A Year's Botany' 1874 page
118.) If a bee with no pollen on its back enters a flower it touches the
stigma, which quickly closes, and when the bee retires dusted with
pollen, it can leave none on the stigma of the same flower. But as soon
as it enters any other flower, plenty of pollen is left on the stigma,
which will be thus cross-fertilised. Nevertheless, if insects are
excluded, the flowers fertilise themselves perfectly and produce plenty
of seed; but I did not ascertain whether this is effected by the stamens
increasing in length with advancing age, or by the bending down of the
pistil. The chief interest in my experiments on the present species,
lies in the appearance in the fourth self-fertilised generation of a
variety which bore large peculiarly-coloured flowers, and grew to a
greater height than the other varieties; it likewise became more highly
self-fertile, so that this variety resembles the plant named Hero, which
appeared in the sixth self-fertilised generation of Ipomoea.
Some flowers on one of the plants raised from the purchased seeds were
fertilised with their own pollen; and others on the same plant were
crossed with pollen from a distinct plant. The seeds from twelve
capsules thus produced were placed in separate watch-glasses for
comparison; and those from the six crossed capsules appeared to the eye
hardly more numerous than those from the six self-fertilised capsules.
But when the seeds were weighed, those from the crossed capsules
amounted to 1.02 grain, whilst those from the self-fertilised capsules
were only .81 grain; so that the former were either heavier or more
numerous than the latter, in the ratio of 100 to 79.
CROSSED AND SELF-FERTILISED PLANTS OF THE FIRST GENERATION.
Having ascertained, by leaving crossed and self-fertilised seed on damp
sand, that they germinated simultaneously, both kinds were thickly sown
on opposite sides of a broad and rather shallow pan; so that the two
sets of seedlings, which came up at the same time, were subjected to the
same unfavourable conditions. This was a bad method of treatment, but
this species was one of the first on which I experimented. When the
crossed seedlings were on an average half an inch high, the
self-fertilised ones were only a quarter of an inch high. When grown to
their full height under the above unfavourable conditions, the four
tallest crossed plants averaged 7.62, and the four tallest
self-fertilised 5.87 inches in height; or as 100 to 77. Ten flowers on
the crossed plants were fully expanded before one on the self-fertilised
plants. A few of these plants of both lots were transplanted into a
large pot with plenty of good earth, and the self-fertilised plants, not
now being subjected to severe competition, grew during the following
year as tall as the crossed plants; but from a case which follows it is
doubtful whether they would have long continued equal. Some flowers on
the crossed plants were crossed with pollen from another plant, and the
capsules thus produced contained a rather greater weight of seed than
those on the self-fertilised plants again self-fertilised.
CROSSED AND SELF-FERTILISED PLANTS OF THE SECOND GENERATION.
Seeds from the foregoing plants, fertilised in the manner just stated,
were sown on the opposite sides of a small pot (1) and came up crowded.
The four tallest crossed seedlings, at the time of flowering, averaged 8
inches in height, whilst the four tallest self-fertilised plants
averaged only 4 inches. Crossed seeds were sown by themselves in a
second small pot, and self-fertilised seeds were sown by themselves in a
third small pot so that there was no competition whatever between these
two lots. Nevertheless the crossed plants grew from 1 to 2 inches higher
on an average than the self-fertilised. Both lots looked equally
vigorous, but the crossed plants flowered earlier and more profusely
than the self-fertilised. In Pot 1, in which the two lots competed with
each other, the crossed plants flowered first and produced a large
number of capsules, whilst the self-fertilised produced only nineteen.
The contents of twelve capsules from the crossed flowers on the crossed
plants, and of twelve capsules from self-fertilised flowers on the
self-fertilised plants, were placed in separate watch-glasses for
comparison; and the crossed seeds seemed more numerous by half than the
self-fertilised.
The plants on both sides of Pot 1, after they had seeded, were cut down
and transplanted into a large pot with plenty of good earth, and on the
following spring, when they had grown to a height of between 5 and 6
inches, the two lots were equal, as occurred in a similar experiment in
the last generation. But after some weeks the crossed plants exceeded
the self-fertilised ones on the opposite side of the same pot, though
not nearly to so great a degree as before, when they were subjected to
very severe competition.
CROSSED AND SELF-FERTILISED PLANTS OF THE THIRD GENERATION.
Crossed seeds from the crossed plants, and self-fertilised seeds from
the self-fertilised plants of the last generation, were sown thickly on
opposite sides of a small pot, Number 1. The two tallest plants on each
side were measured after they had flowered, and the two crossed ones
were 12 and 7 1/2 inches, and the two self-fertilised ones 8 and 5 1/2
inches in height; that is, in the ratio of 100 to 69. Twenty flowers on
the crossed plants were again crossed and produced twenty capsules; ten
of which contained 1.33 grain weight of seeds. Thirty flowers on the
self-fertilised plants were again self-fertilised and produced
twenty-six capsules; ten of the best of which (many being very poor)
contained only .87 grain weight of seeds; that is, in the ratio of 100
to 65 by weight.
The superiority of the crossed over the self-fertilised plants was
proved in various ways. Self-fertilised seeds were sown on one side of a
pot, and two days afterwards crossed seeds on the opposite side. The two
lots of seedlings were equal until they were above half an inch high;
but when fully grown the two tallest crossed plants attained a height of
12 1/2 and 8 3/4 inches, whilst the two tallest self-fertilised plants
were only 8 and 5 1/2 inches high.
In a third pot, crossed seeds were sown four days after the
self-fertilised, and the seedlings from the latter had at first, as
might have been expected, an advantage; but when the two lots were
between 5 and 6 inches in height, they were equal, and ultimately the
three tallest crossed plants were 11, 10, and 8 inches, whilst the three
tallest self-fertilised were 12, 8 1/2, and 7 1/2 inches in height. So
that there was not much difference between them, the crossed plants
having an average advantage of only the third of an inch. The plants
were cut down, and without being disturbed were transplanted into a
larger pot. Thus the two lots started fair on the following spring, and
now the crossed plants showed their inherent superiority, for the two
tallest were 13 inches, whilst the two tallest self-fertilised plants
were only 11 and 8 1/2 inches in height; or as 100 to 75. The two lots
were allowed to fertilise themselves spontaneously: the crossed plants
produced a large number of capsules, whilst the self-fertilised produced
very few and poor ones. The seeds from eight of the capsules on the
crossed plants weighed .65 grain, whilst those from eight of the
capsules on the self-fertilised plants weighed only .22 grain; or as 100
to 34.
The crossed plants in the above three pots, as in almost all the
previous experiments, flowered before the self-fertilised. This occurred
even in the third pot in which the crossed seeds were sown four days
after the self-fertilised seeds.
Lastly, seeds of both lots were sown on opposite sides of a large pot in
which a Fuchsia had long been growing, so that the earth was full of
roots. Both lots grew miserably; but the crossed seedlings had an
advantage at all times, and ultimately attained to a height of 3 1/2
inches, whilst the self-fertilised seedlings never exceeded 1 inch. The
several foregoing experiments prove in a decisive manner the superiority
in constitutional vigour of the crossed over the self-fertilised plants.
In the three generations now described and taken together, the average
height of the ten tallest crossed plants was 8.19 inches, and that of
the ten tallest self-fertilised plants 5.29 inches (the plants having
been grown in small pots), or as 100 to 65.
In the next or fourth self-fertilised generation, several plants of a
new and tall variety appeared, which increased in the later
self-fertilised generations, owing to its great self-fertility, to the
complete exclusion of the original kinds. The same variety also appeared
amongst the crossed plants, but as it was not at first regarded with any
particular attention, I know not how far it was used for raising the
intercrossed plants; and in the later crossed generations it was rarely
present. Owing to the appearance of this tall variety, the comparison of
the crossed and self-fertilised plants of the fifth and succeeding
generations was rendered unfair, as all the self-fertilised and only a
few or none of the crossed plants consisted of it. Nevertheless, the
results of the later experiments are in some respects well worth giving.
CROSSED AND SELF-FERTILISED PLANTS OF THE FOURTH GENERATION.
Seeds of the two kinds, produced in the usual way from the two sets of
plants of the third generation, were sown on opposite sides of two pots
(1 and 2); but the seedlings were not thinned enough and did not grow
well. Many of the self-fertilised plants, especially in one of the pots,
consisted of the new and tall variety above referred to, which bore
large and almost white flowers marked with crimson blotches. I will call
it the WHITE VARIETY. I believe that it first appeared amongst both the
crossed and self-fertilised plants of the last generation; but neither
my gardener nor myself could remember any such variety in the seedlings
raised from the purchased seed. It must therefore have arisen either
through ordinary variation, or, judging from its appearance amongst both
the crossed and self-fertilised plants, more probably through reversion
to a formerly existing variety.
In Pot 1 the tallest crossed plant was 8 1/2 inches, and the tallest
self-fertilised 5 inches in height. In Pot 2, the tallest crossed plant
was 6 1/2 inches, and the tallest self-fertilised plant, which consisted
of the white variety, 7 inches in height; and this was the first
instance in my experiments on Mimulus in which the tallest
self-fertilised plant exceeded the tallest crossed. Nevertheless, the
two tallest crossed plants taken together were to the two tallest
self-fertilised plants in height as 100 to 80. As yet the crossed plants
were superior to the self-fertilised in fertility; for twelve flowers on
the crossed plants were crossed and yielded ten capsules, the seeds of
which weighed 1.71 grain. Twenty flowers on the self-fertilised plants
were self-fertilised, and produced fifteen capsules, all appearing poor;
and the seeds from ten of them weighed only .68 grain, so that from an
equal number of capsules the crossed seeds were to the self-fertilised
in weight as 100 to 40.
CROSSED AND SELF-FERTILISED PLANTS OF THE FIFTH GENERATION.
Seeds from both lots of the fourth generation, fertilised in the usual
manner, were sown on opposite sides of three pots. When the seedlings
flowered, most of the self-fertilised plants were found to consist of
the tall white variety. Several of the crossed plants in Pot 1 likewise
belonged to this variety, as did a very few in Pots 2 and 3. The tallest
crossed plant in Pot 1 was 7 inches, and the tallest self-fertilised
plant on the opposite side 8 inches; in Pots 2 and 3 the tallest crossed
were 4 1/2 and 5 1/2, and the tallest self-fertilised 7 and 6 1/2 inches
in height; so that the average height of the tallest plants in the two
lots was as 100 for the crossed to 126 for the self-fertilised; and thus
we have a complete reversal of what occurred in the four previous
generations. Nevertheless, in all three pots the crossed plants retained
their habit of flowering before the self-fertilised. The plants were
unhealthy from being crowded and from the extreme heat of the season,
and were in consequence more or less sterile; but the crossed plants
were somewhat less sterile than the self-fertilised plants.
CROSSED AND SELF-FERTILISED PLANTS OF THE SIXTH GENERATION.
Seeds from plants of the fifth generation crossed and self-fertilised in
the usual manner were sown on opposite sides of several pots. On the
self-fertilised side every single plant belonged to the tall white
variety. On the crossed side some plants belonged to this variety, but
the greater number approached in character to the old and shorter kinds
with smaller yellowish flowers blotched with coppery brown. When the
plants on both sides were from 2 to 3 inches in height they were equal,
but when fully grown the self-fertilised were decidedly the tallest and
finest plants, but, from want of time, they were not actually measured.
In half the pots the first plant which flowered was a self-fertilised
one, and in the other half a crossed one. And now another remarkable
change was clearly perceived, namely, that the self-fertilised plants
had become more self-fertile than the crossed. The pots were all put
under a net to exclude insects, and the crossed plants produced
spontaneously only fifty-five capsules, whilst the self-fertilised
plants produced eighty-one capsules, or as 100 to 147. The seeds from
nine capsules of both lots were placed in separate watch-glasses for
comparison, and the self-fertilised appeared rather the more numerous.
Besides these spontaneously self-fertilised capsules, twenty flowers on
the crossed plants again crossed yielded sixteen capsules; twenty-five
flowers on the self-fertilised plants again self-fertilised yielded
seventeen capsules, and this is a larger proportional number of capsules
than was produced by the self-fertilised flowers on the self-fertilised
plants in the previous generations. The contents of ten capsules of both
these lots were compared in separate watch-glasses, and the seeds from
the self-fertilised appeared decidedly more numerous than those from the
crossed plants.
CROSSED AND SELF-FERTILISED PLANTS OF THE SEVENTH GENERATION.
Crossed and self-fertilised seeds from the crossed and self-fertilised
plants of the sixth generation were sown in the usual manner on opposite
sides of three pots, and the seedlings were well and equally thinned.
Every one of the self-fertilised plants (and many were raised) in this,
as well as in the eighth and ninth generations, belonged to the tall
white variety. Their uniformity of character, in comparison with the
seedlings first raised from the purchased seed, was quite remarkable. On
the other hand, the crossed plants differed much in the tints of their
flowers, but not, I think, to so great a degree as those first raised. I
determined this time to measure the plants on both sides carefully. The
self-fertilised seedlings came up rather before the crossed, but both
lots were for a time of equal height. When first measured, the average
height of the six tallest crossed plants in the three pots was 7.02, and
that of the six tallest self-fertilised plants 8.97 inches, or as 100 to
128. When fully grown the same plants were again measured, with the
result shown in Table 3/18.
TABLE 3/18. Mimulus luteus (Seventh Generation).
Heights of Plants in inches:
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 1 : 11 2/8 : 19 1/8.
Pot 1 : 11 7/8 : 18.
Pot 2 : 12 6/8 : 18 2/8.
Pot 2 : 11 2/8 : 14 6/8.
Pot 3 : 9 6/8 : 12 6/8.
Pot 3 : 11 6/8 : 11.
Total : 68.63 : 93.88.
The average height of the six crossed is here 11.43, and that of the six
self-fertilised 15.64, or as 100 to 137.
As it is now evident that the tall white variety transmitted its
characters faithfully, and as the self-fertilised plants consisted
exclusively of this variety, it was manifest that they would always
exceed in height the crossed plants which belonged chiefly to the
original shorter varieties. This line of experiment was therefore
discontinued, and I tried whether intercrossing two self-fertilised
plants of the sixth generation, growing in distinct pots, would give
their offspring any advantage over the offspring of flowers on one of
the same plants fertilised with their own pollen. These latter seedlings
formed the seventh generation of self-fertilised plants, like those in
the right hand column in Table 3/18; the crossed plants were the product
of six previous self-fertilised generations with an intercross in the
last generation. The seeds were allowed to germinate on sand, and were
planted in pairs on opposite sides of four pots, all the remaining seeds
being sown crowded on opposite sides of Pot 5 in Table 3/19; the three
tallest on each side in this latter pot being alone measured. All the
plants were twice measured--the first time whilst young, and the average
height of the crossed plants to that of the self-fertilised was then as
100 to 122. When fully grown they were again measured, as in Table 3/19.
TABLE 3/19. Mimulus luteus.
Heights of Plants in inches:
Column 1: Number (Name) of Pot.
Column 2: Intercrossed Plants from Self-fertilised Plants of the Sixth
Generation.
Column 3: Self-fertilised Plants of the Seventh Generation.
Pot 1 : 12 6/8 : 15 2/8.
Pot 1 : 10 4/8 : 11 5/8.
Pot 1 : 10 : 11.
Pot 1 : 14 5/8 : 11.
Pot 2 : 10 2/8 : 11 3/8.
Pot 2 : 7 6/8 : 11 4/8.
Pot 2 : 12 1/8 : 8 5/8.
Pot 2 : 7 : 14 3/8.
Pot 3 : 13 5/8 : 10 3/8.
Pot 3 : 12 2/8 : 11 6/8.
Pot 4 : 7 1/8 : 14 6/8.
Pot 4 : 8 2/8 : 7.
Pot 4 : 7 2/8 : 8.
Pot 5 : 8 5/8 : 10 2/8
Pot 5 : 9 : 9 3/8.
Pot 5 : 8 2/8 : 9 2/8.
Crowded.
Total : 159.38 : 175.50.
The average height of the sixteen intercrossed plants is here 9.96
inches, and that of the sixteen self-fertilised plants 10.96, or as 100
to 110; so that the intercrossed plants, the progenitors of which had
been self-fertilised for the six previous generations, and had been
exposed during the whole time to remarkably uniform conditions, were
somewhat inferior in height to the plants of the seventh self-fertilised
generation. But as we shall presently see that a similar experiment made
after two additional generations of self-fertilisation gave a different
result, I know not how far to trust the present one. In three of the
five pots in Table 3/19 a self-fertilised plant flowered first, and in
the other two a crossed plant. These self-fertilised plants were
remarkably fertile, for twenty flowers fertilised with their own pollen
produced no less than nineteen very fine capsules!
THE EFFECTS OF A CROSS WITH A DISTINCT STOCK.
Some flowers on the self-fertilised plants in Pot 4 in Table 3/19 were
fertilised with their own pollen, and plants of the eighth
self-fertilised generation were thus raised, merely to serve as parents
in the following experiment. Several flowers on these plants were
allowed to fertilise themselves spontaneously (insects being of course
excluded), and the plants raised from these seeds formed the ninth
self-fertilised generation; they consisted wholly of the tall white
variety with crimson blotches. Other flowers on the same plants of the
eighth self-fertilised generation were crossed with pollen taken from
another plant of the same lot; so that the seedlings thus raised were
the offspring of eight previous generations of self-fertilisation with
an intercross in the last generation; these I will call the INTERCROSSED
PLANTS. Lastly, other flowers on the same plants of the eighth
self-fertilised generation were crossed with pollen taken from plants
which had been raised from seed procured from a garden at Chelsea. The
Chelsea plants bore yellow flowers blotched with red, but differed in no
other respect. They had been grown out of doors, whilst mine had been
cultivated in pots in the greenhouse for the last eight generations, and
in a different kind of soil. The seedlings raised from this cross with a
wholly different stock may be called the CHELSEA-CROSSED. The three lots
of seeds thus obtained were allowed to germinate on bare sand; and
whenever a seed in all three lots, or in only two, germinated at the
same time, they were planted in pots superficially divided into three or
two compartments. The remaining seeds, whether or not in a state of
germination, were thickly sown in three divisions in a large pot, 10, in
Table 3/20. When the plants had grown to their full height they were
measured, as shown in Table 3/20; but only the three tallest plants in
each of the three divisions in Pot 10 were measured.
TABLE 3/20. Mimulus luteus.
Heights of Plants in inches:
Column 1: Number (Name) of Pot.
Column 2: Plants from Self-fertilised Plants of the Eighth Generation
crossed by Chelsea Plants.
Column 3: Plants from an intercross between the Plants of the Eighth
Self-fertilised Generation.
Column 4: Self-fertilised Plants of the Ninth Generation from Plants of
the Eighth Self-fertilised Generation.
Pot 1 : 30 7/8 : 14 : 9 4/8.
Pot 1 : 28 3/8 : 13 6/8 : 10 5/8.
Pot 1 : -- : 13 7/8 : 10.
Pot 2 : 20 6/8 : 11 4/8 : 11 6/8.
Pot 2 : 22 2/8 : 12 : 12 3/8.
Pot 2 : -- : 9 1/8 : --.
Pot 3 : 23 6/8 : 12 2/8 : 8 5/8.
Pot 3 : 24 1/8 : -- : 11 4/8.
Pot 3 : 25 6/8 : -- : 6 7/8.
Pot 4 : 22 5/8 : 9 2/8 : 4.
Pot 4 : 22 : 8 1/8 : 13 3/8.
Pot 4 : 17 : -- : 11.
Pot 5 : 22 3/8 : 9 : 4 4/8.
Pot 5 : 19 5/8 : 11 : 13.
Pot 5 : 23 4/8 : -- : 13 4/8.
Pot 6 : 28 2/8 : 18 6/8 : 12.
Pot 6 : 22 : 7 : 16 1/8.
Pot 6 : -- : 12 4/8 : --.
Pot 7 : 12 4/8 : 15 : --.
Pot 7 : 24 3/8 : 12 3/8 : --.
Pot 7 : 20 4/8 : 11 2/8 : --.
Pot 7 : 26 4/8 : 15 2/8 : --.
Pot 8 : 17 2/8 : 13 3/8 : --.
Pot 8 : 22 6/8 : 14 5/8 : --.
Pot 8 : 27 : 14 3/8 : --.
Pot 9 : 22 6/8 : 11 6/8 : --.
Pot 9 : 6 : 17 : --.
Pot 9 : 20 2/8 : 14 7/8 : --.
Pot 10 : 18 1/8 : 9 2/8 : 10 3/8.
Pot 10 : 16 5/8 : 8 2/8 : 8 1/8.
Pot 10 : 17 4/8 : 10 : 11 2/8.
Crowded plants.
Total : 605.38 : 329.50 : 198.50.
In this table the average height of the twenty-eight Chelsea-crossed
plants is 21.62 inches; that of the twenty-seven intercrossed plants
12.2; and that of the nineteen self-fertilised 10.44. But with respect
to the latter it will be the fairest plan to strike out two dwarfed ones
(only 4 inches in height), so as not to exaggerate the inferiority of
the self-fertilised plants; and this will raise the average height of
the seventeen remaining self-fertilised plants to 11.2 inches. Therefore
the Chelsea-crossed are to the intercrossed in height as 100 to 56; the
Chelsea-crossed to the self-fertilised as 100 to 52; and the
intercrossed to the self-fertilised as 100 to 92. We thus see how
immensely superior in height the Chelsea-crossed are to the intercrossed
and to the self-fertilised plants. They began to show their superiority
when only one inch high. They were also, when fully grown, much more
branched with larger leaves and somewhat larger flowers than the plants
of the other two lots, so that if they had been weighed, the ratio would
certainly have been much higher than that of 100 to 56 and 52.
The intercrossed plants are here to the self-fertilised in height as 100
to 92; whereas in the analogous experiment given in Table 3/19 the
intercrossed plants from the self-fertilised plants of the sixth
generation were inferior in height to the self-fertilised plants in the
ratio of 100 to 110. I doubt whether this discordance in the results of
the two experiments can be explained by the self-fertilised plants in
the present case having been raised from spontaneously self-fertilised
seeds, whereas in the former case they were raised from artificially
self-fertilised seeds; nor by the present plants having been
self-fertilised during two additional generations, though this is a more
probable explanation.
With respect to fertility, the twenty-eight Chelsea-crossed plants
produced 272 capsules; the twenty-seven intercrossed plants produced 24;
and the seventeen self-fertilised plants 17 capsules. All the plants
were left uncovered so as to be naturally fertilised, and empty capsules
were rejected.
Therefore 20 Chelsea-crossed plants would have produced 194.29 capsules.
Therefore 20 Intercrossed plants would have produced 17.77 capsules.
Therefore 20 Self-fertilised plants would have produced 20.00 capsules.
The seeds contained in 8 capsules from the Chelsea-crossed plants
weighed 1.1 grains.
The seeds contained in 8 capsules from the Intercrossed plants weighed
0.51 grains.
The seeds contained in 8 capsules from the Self-fertilised plants
weighed 0.33 grains.
If we combine the number of capsules produced together with the average
weight of contained seeds, we get the following extraordinary ratios:
Weight of seed produced by the same number of Chelsea-crossed and
intercrossed plants as 100 to 4.
Weight of seed produced by the same number of Chelsea-crossed and
self-fertilised plants as 100 to 3.
Weight of seeds produced by the same number of intercrossed and
self-fertilised plants as 100 to 73.
It is also a remarkable fact that the Chelsea-crossed plants exceeded
the two other lots in hardiness, as greatly as they did in height,
luxuriance, and fertility. In the early autumn most of the pots were
bedded out in the open ground; and this always injures plants which have
been long kept in a warm greenhouse. All three lots consequently
suffered greatly, but the Chelsea-crossed plants much less than the
other two lots. On the 3rd of October the Chelsea-crossed plants began
to flower again, and continued to do so for some time; whilst not a
single flower was produced by the plants of the other two lots, the
stems of which were cut almost down to the ground and seemed half dead.
Early in December there was a sharp frost, and the stems of
Chelsea-crossed were now cut down; but on the 23rd of December they
began to shoot up again from the roots, whilst all the plants of the
other two lots were quite dead.
Although several of the self-fertilised seeds, from which the plants in
the right hand column in Table 3/20 were raised, germinated (and were of
course rejected) before any of those of the other two lots, yet in only
one of the ten pots did a self-fertilised plant flower before the
Chelsea-crossed or the intercrossed plants growing in the same pots. The
plants of these two latter lots flowered at the same time, though the
Chelsea-crossed grew so much taller and more vigorously than the
intercrossed.
As already stated, the flowers of the plants originally raised from the
Chelsea seeds were yellow; and it deserves notice that every one of the
twenty-eight seedlings raised from the tall white variety fertilised,
without being castrated, with pollen from the Chelsea plants, produced
yellow flowers; and this shows how prepotent this colour, which is the
natural one of the species, is over the white colour.
THE EFFECTS ON THE OFFSPRING OF INTERCROSSING FLOWERS ON THE SAME PLANT,
INSTEAD OF CROSSING DISTINCT INDIVIDUALS.
In all the foregoing experiments the crossed plants were the product of
a cross between distinct plants. I now selected a very vigorous plant in
Table 3/20, raised by fertilising a plant of the eighth self-fertilised
generation with pollen from the Chelsea stock. Several flowers on this
plant were crossed with pollen from other flowers on the same plant, and
several other flowers were fertilised with their own pollen. The seed
thus produced was allowed to germinate on bare sand; and the seedlings
were planted in the usual manner on the opposite sides of six pots. All
the remaining seeds, whether or not in a state of germination, were sown
thickly in Pot 7; the three tallest plants on each side of this latter
pot being alone measured. As I was in a hurry to learn the result, some
of these seeds were sown late in the autumn, but the plants grew so
irregularly during the winter, that one crossed plant was 28 1/2 inches,
and two others only 4, or less than 4 inches in height, as may be seen
in Table 3/21. Under such circumstances, as I have observed in many
other cases, the result is not in the least trustworthy; nevertheless I
feel bound to give the measurements.
TABLE 3/21. Mimulus luteus.
Heights of Plants in inches:
Column 1: Number (Name) of Pot.
Column 2: Plants raised from a Cross between different Flowers on the
same Plant.
Column 3: Plants raised from Flowers fertilised with their own Pollen.
Pot 1 : 17 : 17.
Pot 1 : 9 : 3 1/8.
Pot 2 : 28 2/8 : 19 1/8.
Pot 2 : 16 4/8 : 6.
Pot 2 : 13 5/8 : 2.
Pot 3 : 4 : 15 6/8.
Pot 3 : 2 2/8 : 10.
Pot 4 : 23 4/8 : 6 2/8.
Pot 4 : 15 4/8 : 7 1/8.
Pot 5 : 7 : 13 4/8.
Pot 6 : 18 3/8 : 1 4/8.
Pot 6 : 11 : 2.
Pot 7 : 21 : 15 1/8.
Pot 7 : 11 6/8 : 11.
Pot 7 : 12 1/8 : 11 2/8.
Crowded.
Total : 210.88 : 140.75.
The fifteen crossed plants here average 14.05, and the fifteen
self-fertilised plants 9.38 in height, or as 100 to 67. But if all the
plants under ten inches in height are struck out, the ratio of the
eleven crossed plants to the eight self-fertilised plants is as 100 to
82.
On the following spring, some remaining seeds of the two lots were
treated in exactly the same manner; and the measurements of the
seedlings are given in Table 3/22.
TABLE 3/22. Mimulus luteus.
Heights of Plants in inches:
Column 1: Number (Name) of Pot.
Column 2: Plants raised from a Cross between different Flowers on the
same Plant.
Column 3: Plants raised from Flowers fertilised with their own Pollen.
Pot 1 : 15 1/8 : 19 1/8.
Pot 1 : 12 : 20 5/8.
Pot 1 : 10 1/8 : 12 6/8.
Pot 2 : 16 2/8 : 11 2/8.
Pot 2 : 13 5/8 : 19 3/8.
Pot 2 : 20 1/8 : 17 4/8.
Pot 3 : 18 7/8 : 12 6/8.
Pot 3 : 15 : 15 6/8.
Pot 3 : 13 7/8 : 17.
Pot 4 : 19 2/8 : 16 2/8.
Pot 4 : 19 6/8 : 21 5/8.
Pot 5 : 25 3/8 : 22 5/8.
Pot 6 : 15 : 19 5/8.
Pot 6 : 20 2/8 : 16 2/8.
Pot 6 : 27 2/8 : 19 5/8.
Pot 7 : 7 6/8 : 7 6/8.
Pot 7 : 14 : 8.
Pot 7 : 13 4/8 : 7.
Pot 8 : 18 2/8 : 20 3/8.
Pot 8 : 18 6/8 : 17 6/8.
Pot 8 : 18 3/8 : 15 4/8.
Pot 8 : 18 3/8 : 15 1/8.
Crowded.
Total : 370.88 : 353.63.
Here the average height of the twenty-two crossed plants is 16.85, and
that of the twenty-two self-fertilised plants 16.07; or as 100 to 95.
But if four of the plants in Pot 7, which are much shorter than any of
the others, are struck out (and this would be the fairest plan), the
twenty-one crossed are to the nineteen self-fertilised plants in height
as 100 to 100.6--that is, are equal. All the plants, except the crowded
ones in Pot 8, after being measured were cut down, and the eighteen
crossed plants weighed 10 ounces, whilst the same number of
self-fertilised plants weighed 10 1/4 ounces, or as 100 to 102.5; but if
the dwarfed plants in Pot 7 had been excluded, the self-fertilised would
have exceeded the crossed in weight in a higher ratio. In all the
previous experiments in which seedlings were raised from a cross between
distinct plants, and were put into competition with self-fertilised
plants, the former generally flowered first; but in the present case, in
seven out of the eight pots a self-fertilised plant flowered before a
crossed one on the opposite side. Considering all the evidence with
respect to the plants in Table3/ 22, a cross between two flowers on the
same plant seems to give no advantage to the offspring thus produced,
the self-fertilised plants being in weight superior. But this conclusion
cannot be absolutely trusted, owing to the measurements given in Table
3/21, though these latter, from the cause already assigned, are very
much less trustworthy than the present ones.]
SUMMARY OF OBSERVATIONS ON Mimulus luteus.
In the three first generations of crossed and self-fertilised plants,
the tallest plants alone on each side of the several pots were measured;
and the average height of the ten crossed to that of the ten
self-fertilised plants was as 100 to 64. The crossed were also much more
fertile than the self-fertilised, and so much more vigorous that they
exceeded them in height, even when sown on the opposite side of the same
pot after an interval of four days. The same superiority was likewise
shown in a remarkable manner when both kinds of seeds were sown on the
opposite sides of a pot with very poor earth full of the roots of
another plant. In one instance crossed and self-fertilised seedlings,
grown in rich soil and not put into competition with each other,
attained to an equal height. When we come to the fourth generation the
two tallest crossed plants taken together exceeded by only a little the
two tallest self-fertilised plants, and one of the latter beat its
crossed opponent,--a circumstance which had not occurred in the previous
generations. This victorious self-fertilised plant consisted of a new
white-flowered variety, which grew taller than the old yellowish
varieties. From the first it seemed to be rather more fertile, when
self-fertilised, than the old varieties, and in the succeeding
self-fertilised generations became more and more self-fertile. In the
sixth generation the self-fertilised plants of this variety compared
with the crossed plants produced capsules in the proportion of 147 to
100, both lots being allowed to fertilise themselves spontaneously. In
the seventh generation twenty flowers on one of these plants
artificially self-fertilised yielded no less than nineteen very fine
capsules!
This variety transmitted its characters so faithfully to all the
succeeding self-fertilised generations, up to the last or ninth, that
all the many plants which were raised presented a complete uniformity of
character; thus offering a remarkable contrast with the seedlings raised
from the purchased seeds. Yet this variety retained to the last a latent
tendency to produce yellow flowers; for when a plant of the eighth
self-fertilised generation was crossed with pollen from a
yellow-flowered plant of the Chelsea stock, every single seedling bore
yellow flowers. A similar variety, at least in the colour of its
flowers, also appeared amongst the crossed plants of the third
generation. No attention was at first paid to it, and I know not how far
it was at first used either for crossing or self-fertilisation. In the
fifth generation most of the self-fertilised plants, and in the sixth
and all the succeeding generations every single plant consisted of this
variety; and this no doubt was partly due to its great and increasing
self-fertility. On the other hand, it disappeared from amongst the
crossed plants in the later generations; and this was probably due to
the continued intercrossing of the several plants. From the tallness of
this variety, the self-fertilised plants exceeded the crossed plants in
height in all the generations from the fifth to the seventh inclusive;
and no doubt would have done so in the later generations, had they been
grown in competition with one another. In the fifth generation the
crossed plants were in height to the self-fertilised, as 100 to 126; in
the sixth, as 100 to 147; and in the seventh generation, as 100 to 137.
This excess of height may be attributed not only to this variety
naturally growing taller than the other plants, but to its possessing a
peculiar constitution, so that it did not suffer from continued
self-fertilisation.
This variety presents a strikingly analogous case to that of the plant
called the Hero, which appeared in the sixth self-fertilised generation
of Ipomoea. If the seeds produced by Hero had been as greatly in excess
of those produced by the other plants, as was the case with Mimulus, and
if all the seeds had been mingled together, the offspring of Hero would
have increased to the entire exclusion of the ordinary plants in the
later self-fertilised generations, and from naturally growing taller
would have exceeded the crossed plants in height in each succeeding
generation.
Some of the self-fertilised plants of the sixth generation were
intercrossed, as were some in the eighth generation; and the seedlings
from these crosses were grown in competition with self-fertilised plants
of the two corresponding generations. In the first trial the
intercrossed plants were less fertile than the self-fertilised, and less
tall in the ratio of 100 to 110. In the second trial, the intercrossed
plants were more fertile than the self-fertilised in the ratio of 100 to
73, and taller in the ratio of 100 to 92. Notwithstanding that the
self-fertilised plants in the second trial were the product of two
additional generations of self-fertilisation, I cannot understand this
discordance in the results of the two analogous experiments.
The most important of all the experiments on Mimulus are those in which
flowers on plants of the eighth self-fertilised generation were again
self-fertilised; other flowers on distinct plants of the same lot were
intercrossed; and others were crossed with a new stock of plants from
Chelsea. The Chelsea-crossed seedlings were to the intercrossed in
height as 100 to 56, and in fertility as 100 to 4; and they were to the
self-fertilised plants, in height as 100 to 52, and in fertility as 100
to 3. These Chelsea-crossed plants were also much more hardy than the
plants of the other two lots; so that altogether the gain from the cross
with a fresh stock was wonderfully great.
Lastly, seedlings raised from a cross between flowers on the same plant
were not superior to those from flowers fertilised with their own
pollen; but this result cannot be absolutely trusted, owing to some
previous observations, which, however, were made under very unfavourable
circumstances.
[Digitalis purpurea.
The flowers of the common Foxglove are proterandrous; that is, the
pollen is mature and mostly shed before the stigma of the same flower is
ready for fertilisation. This is effected by the larger humble-bees,
which, whilst in search of nectar, carry pollen from flower to flower.
The two upper and longer stamens shed their pollen before the two lower
and shorter ones. The meaning of this fact probably is, as Dr. Ogle
remarks, that the anthers of the longer stamens stand near to the
stigma, so that they would be the most likely to fertilise it (3/3.
'Popular Science Review' January 1870 page 50.); and as it is an
advantage to avoid self-fertilisation, they shed their pollen first,
thus lessening the chance. There is, however, but little danger of
self-fertilisation until the bifid stigma opens; for Hildebrand found
that pollen placed on the stigma before it had opened produced no
effect. (3/4. 'Geschlechter-Vertheilung bei den Pflanzen' 1867 page 20.)
The anthers, which are large, stand at first transversely with respect
to the tubular corolla, and if they were to dehisce in this position
they would, as Dr. Ogle also remarks, smear with pollen the whole back
and sides of an entering humble-bee in a useless manner; but the anthers
twist round and place themselves longitudinally before they dehisce. The
lower and inner side of the mouth of the corolla is thickly clothed with
hairs, and these collect so much of the fallen pollen that I have seen
the under surface of a humble-bee thickly dusted with it; but this can
never be applied to the stigma, as the bees in retreating do not turn
their under surfaces upwards. I was therefore puzzled whether these
hairs were of any use; but Mr. Belt has, I think, explained their use:
the smaller kinds of bees are not fitted to fertilise the flowers, and
if they were allowed to enter easily they would steal much nectar, and
fewer large bees would haunt the flowers. Humble-bees can crawl into the
dependent flowers with the greatest ease, using the "hairs as footholds
while sucking the honey; but the smaller bees are impeded by them, and
when, having at length struggled through them, they reach the slippery
precipice above, they are completely baffled." Mr. Belt says that he
watched many flowers during a whole season in North Wales, and "only
once saw a small bee reach the nectary, though many were seen trying in
vain to do so." (3/5. 'The Naturalist in Nicaragua' 1874 page 132. But
it appears from H. Muller 'Die Befruchtung der Blumen' 1873 page 285,
that small insects sometimes succeed in entering the flowers.)
I covered a plant growing in its native soil in North Wales with a net,
and fertilised six flowers each with its own pollen, and six others with
pollen from a distinct plant growing within the distance of a few feet.
The covered plant was occasionally shaken with violence, so as to
imitate the effects of a gale of wind, and thus to facilitate as far as
possible self-fertilisation. It bore ninety-two flowers (besides the
dozen artificially fertilised), and of these only twenty-four produced
capsules; whereas almost all the flowers on the surrounding uncovered
plants were fruitful. Of the twenty-four spontaneously self-fertilised
capsules, only two contained their full complement of seed; six
contained a moderate supply; and the remaining sixteen extremely few
seeds. A little pollen adhering to the anthers after they had dehisced,
and accidentally falling on the stigma when mature, must have been the
means by which the above twenty-four flowers were partially
self-fertilised; for the margins of the corolla in withering do not curl
inwards, nor do the flowers in dropping off turn round on their axes, so
as to bring the pollen-covered hairs, with which the lower surface is
clothed, into contact with the stigma--by either of which means
self-fertilisation might be effected.
Seeds from the above crossed and self-fertilised capsules, after
germinating on bare sand, were planted in pairs on the opposite sides of
five moderately-sized pots, which were kept in the greenhouse. The
plants after a time appeared starved, and were therefore, without being
disturbed, turned out of their pots, and planted in the open ground in
two close parallel rows. They were thus subjected to tolerably severe
competition with one another; but not nearly so severe as if they had
been left in the pots. At the time when they were turned out, their
leaves were between 5 and 8 inches in length, and the longest leaf on
the finest plant on each side of each pot was measured, with the result
that the leaves of the crossed plants exceeded, on an average, those of
the self-fertilised plants by .4 of an inch.
In the following summer the tallest flower-stem on each plant, when
fully grown, was measured. There were seventeen crossed plants; but one
did not produce a flower-stem. There were also, originally, seventeen
self-fertilised plants, but these had such poor constitutions that no
less than nine died in the course of the winter and spring, leaving only
eight to be measured, as in Table 3/23.
TABLE 3/23. Digitalis purpurea.
The tallest Flower-stem on each Plant measured in inches: 0 means that
the Plant died before a Flower-stem was produced.
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 1 : 53 6/8 : 27 4/8.
Pot 1 : 57 4/8 : 55 6/8.
Pot 1 : 57 6/8 : 0.
Pot 1 : 65 : 0.
Pot 2 : 34 4/8 : 39.
Pot 2 : 52 4/8 : 32.
Pot 2 : 63 6/8 : 21.
Pot 3 : 57 4/8 : 53 4/8.
Pot 3 : 53 4/8 : 0.
Pot 3 : 50 6/8 : 0.
Pot 3 : 37 2/8 : 0.
Pot 4 : 64 4/8 : 34 4/8.
Pot 4 : 37 4/8 : 23 6/8.
Pot 4 : -- : 0.
Pot 5 : 53 : 0.
Pot 5 : 47 6/8 : 0.
Pot 5 : 34 6/8 : 0.
Total : 821.25 : 287.00.
The average height of the flower-stems of the sixteen crossed plants is
here 51.33 inches; and that of the eight self-fertilised plants, 35.87;
or as 100 to 70. But this difference in height does not give at all a
fair idea of the vast superiority of the crossed plants. These latter
produced altogether sixty-four flower-stems, each plant producing, on an
average, exactly four flower-stems, whereas the eight self-fertilised
plants produced only fifteen flower-stems, each producing an average
only of 1.87 stems, and these had a less luxuriant appearance. We may
put the result in another way: the number of flower-stems on the crossed
plants was to those on an equal number of self-fertilised plants as 100
to 48.
Three crossed seeds in a state of germination were also planted in three
separate pots; and three self-fertilised seeds in the same state in
three other pots. These plants were therefore at first exposed to no
competition with one another, and when turned out of their pots into the
open ground they were planted at a moderate distance apart, so that they
were exposed to much less severe competition than in the last case. The
longest leaves on the three crossed plants, when turned out, exceeded
those on the self-fertilised plants by a mere trifle, namely, on an
average by .17 of an inch. When fully grown the three crossed plants
produced twenty-six flower-stems; the two tallest of which on each plant
were on an average 54.04 inches in height. The three self-fertilised
plants produced twenty-three flower-stems, the two tallest of which on
each plant had an average height of 46.18 inches. So that the difference
between these two lots, which hardly competed together, is much less
than in the last case when there was moderately severe competition,
namely, as 100 to 85, instead of as 100 to 70.
THE EFFECTS ON THE OFFSPRING OF INTERCROSSING DIFFERENT FLOWERS ON THE
SAME PLANT, INSTEAD OF CROSSING DISTINCT INDIVIDUALS.
A fine plant growing in my garden (one of the foregoing seedlings) was
covered with a net, and six flowers were crossed with pollen from
another flower on the same plant, and six others were fertilised with
their own pollen. All produced good capsules. The seeds from each were
placed in separate watch-glasses, and no difference could be perceived
by the eye between the two lots of seeds; and when they were weighed
there was no difference of any significance, as the seeds from the
self-fertilised capsules weighed 7.65 grains, whilst those from the
crossed capsules weighed 7.7 grains. Therefore the sterility of the
present species, when insects are excluded, is not due to the impotence
of pollen on the stigma of the same flower. Both lots of seeds and
seedlings were treated in exactly the same manner as in Table 3/23,
excepting that after the pairs of germinating seeds had been planted on
the opposite sides of eight pots, all the remaining seeds were thickly
sown on the opposite sides of Pots 9 and 10 in Table 3/24. The young
plants during the following spring were turned out of their pots,
without being disturbed, and planted in the open ground in two rows, not
very close together, so that they were subjected to only moderately
severe competition with one another. Very differently to what occurred
in the first experiment, when the plants were subjected to somewhat
severe mutual competition, an equal number on each side either died or
did not produce flower-stems. The tallest flower-stems on the surviving
plants were measured, as shown in Table 3/24.
TABLE 3/24. Digitalis purpurea.
The tallest Flower-stem on each Plant measured in inches: 0 signifies
that the Plant died, or did not produce a Flower-stem.
Column 1: Number (Name) of Pot.
Column 2: Plants raised from a Cross between different Flowers on the
same Plant.
Column 3: Plants raised from Flowers fertilised with their own Pollen.
Pot 1 : 49 4/8 : 45 5/8.
Pot 1 : 46 7/8 : 52.
Pot 1 : 43 6/8 : 0.
Pot 2 : 38 4/8 : 54 4/8.
Pot 2 : 47 4/8 : 47 4/8.
Pot 2 : 0 : 32 5/8.
Pot 3 : 54 7/8 : 46 5/8.
Pot 4 : 32 1/8 : 41 3/8.
Pot 4 : 0 : 29 7/8.
Pot 4 : 43 7/8 : 37 1/8.
Pot 5 : 46 6/8 : 42 1/8.
Pot 5 : 40 4/8 : 42 1/8.
Pot 5 : 43 : 0.
Pot 6 : 48 2/8 : 47 7/8.
Pot 6 : 46 2/8 : 48 3/8.
Pot 7 : 48 5/8 : 25.
Pot 7 : 42 : 40 5/8.
Pot 8 : 46 7/8 : 39 1/8.
Pot 9 : 49 : 30 3/8.
Pot 9 : 50 3/8 : 15.
Pot 9 : 46 3/8 : 36 7/8.
Pot 9 : 47 6/8 : 44 1/8.
Pot 9 : 0 : 31 6/8.
Crowded Plants.
Pot 10 : 46 4/8 : 47 7/8.
Pot 10 : 35 2/8 : 0.
Pot 10 : 24 5/8 : 34 7/8.
Pot 10 : 41 4/8 : 40 7/8.
Pot 10 : 17 3/8 : 41 1/8.
Crowded Plants.
Total : 1078.00 : 995.38.
The average height of the flower-stems on the twenty-five crossed plants
in all the pots taken together is 43.12 inches, and that of the
twenty-five self-fertilised plants 39.82, or as 100 to 92. In order to
test this result, the plants planted in pairs in Pots 1 and 8 were
considered by themselves, and the average height of the sixteen crossed
plants is here 44.9, and that of the sixteen self-fertilised plants
42.03, or as 100 to 94. Again, the plants raised from the thickly sown
seed in Pots 9 and 10, which were subjected to very severe mutual
competition, were taken by themselves, and the average height of the
nine crossed plants is 39.86, and that of the nine self-fertilised
plants 35.88, or as 100 to 90. The plants in these two latter pots (9
and 10), after being measured, were cut down close to the ground and
weighed: the nine crossed plants weighed 57.66 ounces, and the nine
self-fertilised plants 45.25 ounces, or as 100 to 78. On the whole we
may conclude, especially from the evidence of weight, that seedlings
from a cross between flowers on the same plant have a decided, though
not great, advantage over those from flowers fertilised with their own
pollen, more especially in the case of the plants subjected to severe
mutual competition. But the advantage is much less than that exhibited
by the crossed offspring of distinct plants, for these exceeded the
self-fertilised plants in height as 100 to 70, and in the number of
flower-stems as 100 to 48. Digitalis thus differs from Ipomoea, and
almost certainly from Mimulus, as with these two species a cross between
flowers on the same plant did no good.
CALCEOLARIA.
A BUSHY GREENHOUSE VARIETY, WITH YELLOW FLOWERS BLOTCHED WITH PURPLE.
The flowers in this genus are constructed so as to favour or almost
ensure cross-fertilisation (3/6. Hildebrand as quoted by H. Muller 'Die
Befruchtung der Blumen' 1873 page 277.); and Mr. Anderson remarks that
extreme care is necessary to exclude insects in order to preserve any
kind true. (3/7. 'Gardeners' Chronicle' 1853 page 534.) He adds the
interesting statement, that when the corolla is cut quite away, insects,
as far as he has seen, never discover or visit the flowers. This plant
is, however, self-fertile if insects are excluded. So few experiments
were made by me, that they are hardly worth giving. Crossed and
self-fertilised seeds were sown on opposite sides of a pot, and after a
time the crossed seedlings slightly exceeded the self-fertilised in
height. When a little further grown, the longest leaves on the former
were very nearly 3 inches in length, whilst those on the self-fertilised
plants were only 2 inches. Owing to an accident, and to the pot being
too small, only one plant on each side grew up and flowered; the crossed
plant was 19 1/2 inches in height, and the self-fertilised one 15
inches; or as 100 to 77.
Linaria vulgaris.
It has been mentioned in the introductory chapter that two large beds of
this plant were raised by me many years ago from crossed and
self-fertilised seeds, and that there was a conspicuous difference in
height and general appearance between the two lots. The trial was
afterwards repeated with more care; but as this was one of the first
plants experimented on, my usual method was not followed. Seeds were
taken from wild plants growing in this neighbourhood and sown in poor
soil in my garden. Five plants were covered with a net, the others being
left exposed to the bees, which incessantly visit the flowers of this
species, and which, according to H. Muller, are the exclusive
fertilisers. This excellent observer remarks that, as the stigma lies
between the anthers and is mature at the same time with them,
self-fertilisation is possible. (3/8. 'Die Befruchtung' etc. page 279.)
But so few seeds are produced by protected plants, that the pollen and
stigma of the same flower seem to have little power of mutual
interaction. The exposed plants bore numerous capsules forming solid
spikes. Five of these capsules were examined and appeared to contain an
equal number of seeds; and these being counted in one capsule, were
found to be 166. The five protected plants produced altogether only
twenty-five capsules, of which five were much finer than all the others,
and these contained an average of 23.6 seeds, with a maximum in one
capsule of fifty-five. So that the number of seeds in the capsules on
the exposed plants to the average number in the finest capsules on the
protected plants was as 100 to 14.
Some of the spontaneously self-fertilised seeds from under the net, and
some seeds from the uncovered plants naturally fertilised and almost
certainly intercrossed by the bees, were sown separately in two large
pots of the same size; so that the two lots of seedlings were not
subjected to any mutual competition. Three of the crossed plants when in
full flower were measured, but no care was taken to select the tallest
plants; their heights were 7 4/8, 7 2/8, and 6 4/8 inches; averaging
7.08 in height. The three tallest of all the self-fertilised plants were
then carefully selected, and their heights were 6 3/8, 5 5/8, and 5 2/8,
averaging 5.75 in height. So that the naturally crossed plants were to
the spontaneously self-fertilised plants in height, at least as much as
100 to 81.
Verbascum thapsus.
The flowers of this plant are frequented by various insects, chiefly by
bees, for the sake of the pollen. Hermann Muller, however, has shown
('Die Befruchtung' etc. page 277) that V. nigrum secretes minute drops
of nectar. The arrangement of the reproductive organs, though not at all
complex, favours cross-fertilisation; and even distinct species are
often crossed, for a greater number of naturally produced hybrids have
been observed in this genus than in almost any other. (3/9. I have given
a striking case of a large number of such hybrids between Verbascum
thapsus and lychnitis found growing wild: 'Journal of Linnean Society
Botany' volume 10 page 451.) Nevertheless the present species is
perfectly self-fertile, if insects are excluded; for a plant protected
by a net was as thickly loaded with fine capsules as the surrounding
uncovered plants. Verbascum lychnitis is rather less self-fertile, for
some protected plants did not yield quite so many capsules as the
adjoining uncovered plants.
Plants of Verbascum thapsus had been raised for a distinct purpose from
self-fertilised seeds; and some flowers on these plants were again
self-fertilised, yielding seed of the second self-fertilised generation;
and other flowers were crossed with pollen from a distinct plant. The
seeds thus produced were sown on the opposite sides of four large pots.
They germinated, however, so irregularly (the crossed seedlings
generally coming up first) that I was able to save only six pairs of
equal age. These when in full flower were measured, as in Table 3/25.
TABLE 3/25. Verbascum thapsus.
Heights of Plants measured in inches.
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants of the Second Generation.
Pot 1 : 76 : 53 4/8.
Pot 2 : 54 : 66.
Pot 3 : 62 : 75.
Pot 3 : 60 5/8 : 30 4/8.
Pot 4 : 73 : 62.
Pot 4 : 66 4/8 : 52.
Total : 392.13 : 339.00.
We here see that two of the self-fertilised plants exceed in height
their crossed opponents. Nevertheless the average height of the six
crossed plants is 65.34 inches, and that of the six self-fertilised
plants 56.5 inches; or as 100 to 86.
Vandellia nummularifolia.
Seeds were sent to me by Mr. J. Scott from Calcutta of this small Indian
weed, which bears perfect and cleistogene flowers. (3/10. The convenient
term of CLEISTOGENE was proposed by Kuhn in an article on the present
genus in 'Bot. Zeitung' 1867 page 65.) The latter are extremely small,
imperfectly developed, and never expand, yet yield plenty of seeds. The
perfect and open flowers are also small, of a white colour with purple
marks; they generally produce seed, although the contrary has been
asserted; and they do so even if protected from insects. They have a
rather complicated structure, and appear to be adapted for
cross-fertilisation, but were not carefully examined by me. They are not
easy to fertilise artificially, and it is possible that some of the
flowers which I thought that I had succeeded in crossing were afterwards
spontaneously self-fertilised under the net. Sixteen capsules from the
crossed perfect flowers contained on an average ninety-three seeds (with
a maximum in one capsule of 137), and thirteen capsules from the
self-fertilised perfect flowers contained sixty-two seeds (with a
maximum in one capsule of 135); or as 100 to 67. But I suspect that this
considerable excess was accidental, as on one occasion nine crossed
capsules were compared with seven self-fertilised capsules (both
included in the above number), and they contained almost exactly the
same average number of seed. I may add that fifteen capsules from
self-fertilised cleistogene flowers contained on an average sixty-four
seeds, with a maximum in one of eighty-seven.
Crossed and self-fertilised seeds from the perfect flowers, and other
seeds from the self-fertilised cleistogene flowers, were sown in five
pots, each divided superficially into three compartments. The seedlings
were thinned at an early age, so that twenty plants were left in each of
the three divisions. The crossed plants when in full flower averaged 4.3
inches, and the self-fertilised plants from the perfect flowers 4.27
inches in height; or as 100 to 99. The self-fertilised plants from the
cleistogene flowers averaged 4.06 inches in height; so that the crossed
were in height to these latter plants as 100 to 94.
I determined to compare again the growth of plants raised from crossed
and self-fertilised perfect flowers, and obtained two fresh lots of
seeds. These were sown on opposite sides of five pots, but they were not
sufficiently thinned, so that they grew rather crowded. When fully
grown, all those above 2 inches in height were selected, all below this
standard being rejected; the former consisted of forty-seven crossed and
forty-one self-fertilised plants; thus a greater number of the crossed
than of the self-fertilised plants grew to a height of above 2 inches.
Of the crossed plants, the twenty-four tallest were on an average 3.6
inches in height; whilst the twenty-four tallest self-fertilised plants
were 3.38 inches in average height; or as 100 to 94. All these plants
were then cut down close to the ground, and the forty-seven crossed
plants weighed 1090.3 grains, and the forty-one self-fertilised plants
weighed 887.4 grains. Therefore an equal number of crossed and
self-fertilised would have been to each other in weight as 100 to 97.
From these several facts we may conclude that the crossed plants had
some real, though very slight, advantage in height and weight over the
self-fertilised plants, when grown in competition with one another.
The crossed plants were, however, inferior in fertility to the
self-fertilised. Six of the finest plants were selected out of the
forty-seven crossed plants, and six out of the forty-one self-fertilised
plants; and the former produced 598 capsules, whilst the latter or
self-fertilised plants produced 752 capsules. All these capsules were
the product of cleistogene flowers, for the plants did not bear during
the whole of this season any perfect flowers. The seeds were counted in
ten cleistogene capsules produced by crossed plants, and their average
number was 46.4 per capsule; whilst the number in ten cleistogene
capsules produced by the self-fertilised plants was 49.4; or as 100 to
106.
3. GESNERIACEAE.--Gesneria pendulina.
In Gesneria the several parts of the flower are arranged on nearly the
same plan as in Digitalis, and most or all of the species are
dichogamous. (3/11. Dr. Ogle 'Popular Science Review' January 1870 page
51.) Plants were raised from seed sent me by Fritz Muller from South
Brazil. Seven flowers were crossed with pollen from a distinct plant,
and produced seven capsules containing by weight 3.01 grains of seeds.
Seven flowers on the same plants were fertilised with their own pollen,
and their seven capsules contained exactly the same weight of seeds.
Germinating seeds were planted on opposite sides of four pots, and when
fully grown measured to the tips of their leaves.
TABLE 3/26. Gesneria pendulina.
Heights of Plants measured in inches.
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 1 : 42 2/8 : 39.
Pot 1 : 24 4/8 : 27 3/8.
Pot 2 : 33 : 30 6/8.
Pot 2 : 27 : 19 2/8.
Pot 3 : 33 4/8 : 31 7/8.
Pot 3 : 29 4/8 : 28 6/8.
Pot 4 : 30 6/8 : 29 6/8.
Pot 4 : 36 : 26 3/8.
Total : 256.50 : 233.13.
The average height of the eight crossed plants is 32.06 inches, and that
of the eight self-fertilised plants 29.14; or as 100 to 90.
4. LABIATAE.--Salvia coccinea. (3/12. The admirable mechanical
adaptations in this genus for favouring or ensuring cross-fertilisation,
have been fully described by Sprengel, Hildebrand, Delpino, H. Muller,
Ogle, and others, in their several works.)
This species, unlike most of the others in the same genus, yields a good
many seeds when insects are excluded. I gathered ninety-eight capsules
produced by flowers spontaneously self-fertilised under a net, and they
contained on an average 1.45 seeds, whilst flowers artificially
fertilised with their own pollen, in which case the stigma will have
received plenty of pollen, yielded on an average 3.3 seeds, or more than
twice as many. Twenty flowers were crossed with pollen from a distinct
plant, and twenty-six were self-fertilised. There was no great
difference in the proportional number of flowers which produced capsules
by these two processes, or in the number of the contained seeds, or in
the weight of an equal number of seeds.
Seeds of both kinds were sown rather thickly on opposite sides of three
pots. When the seedlings were about 3 inches in height, the crossed
showed a slight advantage over the self-fertilised. When two-thirds
grown, the two tallest plants on each side of each pot were measured;
the crossed averaged 16.37 inches, and the self-fertilised 11.75 in
height; or as 100 to 71. When the plants were fully grown and had done
flowering, the two tallest plants on each side were again measured, with
the results shown in Table 3/27.
TABLE 3/27. Salvia coccinea.
Heights of Plants measured in inches.
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 1 : 32 6/8 : 25.
Pot 1 : 20 : 18 6/8.
Pot 2 : 32 3/8 : 20 6/8.
Pot 2 : 24 4/8 : 19 4/8.
Pot 3 : 29 4/8 : 25.
Pot 3 : 28 : 18.
Total : 167.13 : 127.00.
It may be here seen that each of the six tallest crossed plants exceeds
in height its self-fertilised opponent; the former averaged 27.85
inches, whilst the six tallest self-fertilised plants averaged 21.16
inches; or as 100 to 76. In all three pots the first plant which
flowered was a crossed one. All the crossed plants together produced 409
flowers, whilst all the self-fertilised together produced only 232
flowers; or as 100 to 57. So that the crossed plants in this respect
were far more productive than the self-fertilised.
Origanum vulgare.
This plant exists, according to H. Muller, under two forms; one
hermaphrodite and strongly proterandrous, so that it is almost certain
to be fertilised by pollen from another flower; the other form is
exclusively female, has a smaller corolla, and must of course be
fertilised by pollen from a distinct plant in order to yield any seeds.
The plants on which I experimented were hermaphrodites; they had been
cultivated for a long period as a pot-herb in my kitchen garden, and
were, like so many long-cultivated plants, extremely sterile. As I felt
doubtful about the specific name I sent specimens to Kew, and was
assured that the species was Origanum vulgare. My plants formed one
great clump, and had evidently spread from a single root by stolons. In
a strict sense, therefore, they all belonged to the same individual. My
object in experimenting on them was, firstly, to ascertain whether
crossing flowers borne by plants having distinct roots, but all derived
asexually from the same individual, would be in any respect more
advantageous than self-fertilisation; and, secondly, to raise for future
trial seedlings which would constitute really distinct individuals.
Several plants in the above clump were covered by a net, and about two
dozen seeds (many of which, however, were small and withered) were
obtained from the flowers thus spontaneously self-fertilised. The
remainder of the plants were left uncovered and were incessantly visited
by bees, so that they were doubtless crossed by them. These exposed
plants yielded rather more and finer seed (but still very few) than did
the covered plants. The two lots of seeds thus obtained were sown on
opposite sides of two pots; the seedlings were carefully observed from
their first growth to maturity, but they did not differ at any period in
height or in vigour, the importance of which latter observation we shall
presently see. When fully grown, the tallest crossed plant in one pot
was a very little taller than the tallest self-fertilised plant on the
opposite side, and in the other pot exactly the reverse occurred. So
that the two lots were in fact equal; and a cross of this kind did no
more good than crossing two flowers on the same plant of Ipomoea or
Mimulus.
The plants were turned out of the two pots without being disturbed and
planted in the open ground, in order that they might grow more
vigorously. In the following summer all the self-fertilised and some of
the quasi-crossed plants were covered by a net. Many flowers on the
latter were crossed by me with pollen from a distinct plant, and others
were left to be crossed by the bees. These quasi-crossed plants produced
rather more seed than did the original ones in the great clump when left
to the action of the bees. Many flowers on the self-fertilised plants
were artificially self-fertilised, and others were allowed to fertilise
themselves spontaneously under the net, but they yielded altogether very
few seeds. These two lots of seeds--the product of a cross between
distinct seedlings, instead of as in the last case between plants
multiplied by stolons, and the product of self-fertilised flowers--were
allowed to germinate on bare sand, and several equal pairs were planted
on opposite sides of two LARGE pots. At a very early age the crossed
plants showed some superiority over the self-fertilised, which was ever
afterwards retained. When the plants were fully grown, the two tallest
crossed and the two tallest self-fertilised plants in each pot were
measured, as shown in Table 3/28. I regret that from want of time I did
not measure all the pairs; but the tallest on each side seemed fairly to
represent the average difference between the two lots.
TABLE 3/28. Origanum vulgare.
Heights of Plants measured in inches.
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants (two tallest in each pot).
Column 3: Self-fertilised Plants (two tallest in each pot).
Pot 1 : 26 : 24.
Pot 1 : 21 : 21.
Pot 2 : 17 : 12.
Pot 2 : 16 : 11 4/8.
Total : 80.0 : 68.5.
The average height of the crossed plants is here 20 inches, and that of
the self-fertilised 17.12; or as 100 to 86. But this excess of height by
no means gives a fair idea of the vast superiority in vigour of the
crossed over the self-fertilised plants. The crossed flowered first and
produced thirty flower-stems, whilst the self-fertilised produced only
fifteen, or half the number. The pots were then bedded out, and the
roots probably came out of the holes at the bottom and thus aided their
growth. Early in the following summer the superiority of the crossed
plants, owing to their increase by stolons, over the self-fertilised
plants was truly wonderful. In Pot 1, and it should be remembered that
very large pots had been used, the oval clump of crossed plants was 10
by 4 1/2 inches across, with the tallest stem, as yet young, 5 1/2
inches in height; whilst the clump of self-fertilised plants, on the
opposite side of the same pot, was only 3 1/2 by 2 1/2 inches across,
with the tallest young stem 4 inches in height. In Pot 2, the clump of
crossed plants was 18 by 9 inches across, with the tallest young stem 8
1/2 inches in height; whilst the clump of self-fertilised plants on the
opposite side of the same pot was 12 by 4 1/2 inches across, with the
tallest young stem 6 inches in height. The crossed plants during this
season, as during the last, flowered first. Both the crossed and
self-fertilised plants being left freely exposed to the visits of bees,
manifestly produced much more seed than their grand-parents,--the plants
of the original clump still growing close by in the same garden, and
equally left to the action of the bees.
5. ACANTHACEAE.--Thunbergia alata.
It appears from Hildebrand's description ('Botanische Zeitung' 1867 page
285) that the conspicuous flowers of this plant are adapted for
cross-fertilisation. Seedlings were twice raised from purchased seed;
but during the early summer, when first experimented on, they were
extremely sterile, many of the anthers containing hardly any pollen.
Nevertheless, during the autumn these same plants spontaneously produced
a good many seeds. Twenty-six flowers during the two years were crossed
with pollen from a distinct plant, but they yielded only eleven
capsules; and these contained very few seeds! Twenty-eight flowers were
fertilised with pollen from the same flower, and these yielded only ten
capsules, which, however, contained rather more seed than the crossed
capsules. Eight pairs of germinating seeds were planted on opposite
sides of five pots; and exactly half the crossed and half the
self-fertilised plants exceeded their opponents in height. Two of the
self-fertilised plants died young, before they were measured, and their
crossed opponents were thrown away. The six remaining pairs of these
grew very unequally, some, both of the crossed and self-fertilised
plants, being more than twice as tall as the others. The average height
of the crossed plants was 60 inches, and that of the self-fertilised
plants 65 inches, or as 100 to 108. A cross, therefore, between distinct
individuals here appears to do no good; but this result deduced from so
few plants in a very sterile condition and growing very unequally,
obviously cannot be trusted.]
CHAPTER IV.
CRUCIFERAE, PAPAVERACEAE, RESEDACEAE, ETC.
Brassica oleracea, crossed and self-fertilised plants.
Great effect of a cross with a fresh stock on the weight of the
offspring.
Iberis umbellata.
Papaver vagum.
Eschscholtzia californica, seedlings from a cross with a fresh stock not
more vigorous, but more fertile than the self-fertilised seedlings.
Reseda lutea and odorata, many individuals sterile with their own pollen.
Viola tricolor, wonderful effects of a cross.
Adonis aestivalis.
Delphinium consolida.
Viscaria oculata, crossed plants hardly taller, but more fertile than
the self-fertilised.
Dianthus caryophyllus, crossed and self-fertilised plants compared for
four generations.
Great effects of a cross with a fresh stock.
Uniform colour of the flowers on the self-fertilised plants.
Hibiscus africanus.
[6. CRUCIFERAE.--Brassica oleracea.
VAR. CATTELL'S EARLY BARNES CABBAGE.
The flowers of the common cabbage are adapted, as shown by H. Muller,
for cross-fertilisation, and should this fail, for self-fertilisation.
(4/1. 'Die Befruchtung' etc. page 139.) It is well known that the
varieties are crossed so largely by insects, that it is impossible to
raise pure kinds in the same garden, if more than one kind is in flower
at the same time. Cabbages, in one respect, were not well fitted for my
experiments, as, after they had formed heads, they were often difficult
to measure. The flower-stems also differ much in height; and a poor
plant will sometimes throw up a higher stem than that of a fine plant.
In the later experiments, the fully-grown plants were cut down and
weighed, and then the immense advantage from a cross became manifest.
A single plant of the above variety was covered with a net just before
flowering, and was crossed with pollen from another plant of the same
variety growing close by; and the seven capsules thus produced contained
on an average 16.3 seeds, with a maximum of twenty in one capsule. Some
flowers were artificially self-fertilised, but their capsules did not
contain so many seeds as those from flowers spontaneously
self-fertilised under the net, of which a considerable number were
produced. Fourteen of these latter capsules contained on an average 4.1
seeds, with a maximum in one of ten seeds; so that the seeds in the
crossed capsules were in number to those in the self-fertilised capsules
as 100 to 25. The self-fertilised seeds, fifty-eight of which weighed
3.88 grains, were, however, a little finer than those from the crossed
capsules, fifty-eight of which weighed 3.76 grains. When few seeds are
produced, these seem often to be better nourished and to be heavier than
when many are produced.
The two lots of seeds in an equal state of germination were planted,
some on opposite sides of a single pot, and some in the open ground. The
young crossed plants in the pot at first exceeded by a little in height
the self-fertilised; then equalled them; were then beaten; and lastly
were again victorious. The plants, without being disturbed, were turned
out of the pot, and planted in the open ground; and after growing for
some time, the crossed plants, which were all of nearly the same height,
exceeded the self-fertilised ones by 2 inches. When they flowered, the
flower-stems of the tallest crossed plant exceeded that of the tallest
self-fertilised plant by 6 inches. The other seedlings which were
planted in the open ground stood separate, so that they did not compete
with one another; nevertheless the crossed plants certainly grew to a
rather greater height than the self-fertilised; but no measurements were
made. The crossed plants which had been raised in the pot, and those
planted in the open ground, all flowered a little before the
self-fertilised plants.
CROSSED AND SELF-FERTILISED PLANTS OF THE SECOND GENERATION.
Some flowers on the crossed plants of the last generation were again
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