The Different Forms of Flowers on Plants of the Same Species
Part 6 out of 6
M. Duval-Jouve states that the panicles very rarely protrude from their sheaths,
but that when this does happen the flowers expand and exhibit well-developed
ovaries and stigmas, together with full-sized anthers containing apparently
sound pollen; nevertheless such flowers are invariably quite sterile. Schreiber
had previously observed that if a panicle is only half protruded, this half is
sterile, whilst the still included half is fertile. Some plants which grew in a
large tub of water in my greenhouse behaved on one occasion in a very different
manner. They protruded two very large much-branched panicles; but the florets
never opened, though these included fully developed stigmas, and stamens
supported on long filaments with large anthers that dehisced properly. If these
florets had opened for a short time unperceived by me and had then closed again,
the empty anthers would have been left dangling outside. Nevertheless they
yielded on August 17th an abundance of fine ripe seeds. Here then we have a near
approach to the single case as yet known of this grass producing in a state of
nature (in Germany) perfect flowers which yielded a copious supply of fruit.
(8/26. Dr. Ascherson 'Botanische Zeitung' 1864 page 350.) Seeds from the
cleistogamic flowers were sent by me to Mr. Scott in Calcutta, who there
cultivated the plants in various ways, but they never produced perfect flowers.
In Europe Leersia oryzoides is the sole representative of its genus, and Duval-
Jouve, after examining several exotic species, found that it apparently is the
sole one which bears cleistogamic flowers. It ranges from Persia to North
America, and specimens from Pennsylvania resembled the European ones in their
concealed manner of fructification. There can therefore be little doubt that
this plant generally propagates itself throughout an immense area by
cleistogamic seeds, and that it can hardly ever be invigorated by cross-
fertilisation. It resembles in this respect those plants which are now widely
spread, though they increase solely by asexual generation. (8/27. I have
collected several such cases in my 'Variation under Domestication' chapter 18
2nd edition volume 2 page 153.)
CONCLUDING REMARKS ON CLEISTOGAMIC FLOWERS.
That these flowers owe their structure primarily to the arrested development of
perfect ones, we may infer from such cases as that of the lower rudimentary
petal in Viola being larger than the others, like the lower lip of the perfect
flower,--from a vestige of a spur in the cleistogamic flowers of Impatiens,--
from the ten stamens of Ononis being united into a tube,--and other such
structures. The same inference may be drawn from the occurrence, in some
instances, on the same plant of a series of gradations between the cleistogamic
and perfect flowers. But that the former owe their origin wholly to arrested
development is by no means the case; for various parts have been specially
modified, so as to aid in the self-fertilisation of the flowers, and as a
protection to the pollen; for instance, the hook-shaped pistil in Viola and in
some other genera, by which the stigma is brought close to the fertile anthers,-
-the rudimentary corolla of Specularia modified into a perfectly closed
tympanum, and the sheath of Monochoria modified into a closed sack,--the
excessively thin coats of the pollen-grains,--the anthers not being all equally
aborted, and other such cases. Moreover Mr. Bennett has shown that the buds of
the cleistogamic and perfect flowers of Impatiens differ at a very early period
The degree to which many of the most important organs in these degraded flowers
have been reduced or even wholly obliterated, is one of their most remarkable
peculiarities, reminding us of many parasitic animals. In some cases only a
single anther is left, and this contains but few pollen-grains of diminished
size; in other cases the stigma has disappeared, leaving a simple open passage
into the ovarium. It is also interesting to note the complete loss of trifling
points in the structure or functions of certain parts, which though of service
to the perfect flowers, are of none to the cleistogamic; for instance the
collecting hairs on the pistil of Specularia, the glands on the calyx of the
Malpighiaceae, the nectar-secreting appendages to the lower stamens of Viola,
the secretion of nectar by other parts, the emission of a sweet odour, and
apparently the elasticity of the valves in the buried capsules of Viola odorata.
We here see, as throughout nature, that as soon as any part or character becomes
superfluous it tends sooner or later to disappear.
Another peculiarity in these flowers is that the pollen-grains generally emit
their tubes whilst still enclosed within the anthers; but this is not so
remarkable a fact as was formerly thought, when the case of Asclepias was alone
known. (8/28. The case of Asclepias was described by R. Brown. Baillon asserts
'Adansonia' tome 2 1862 page 58, that with many plants the tubes are emitted
from pollen-grains which have not come into contact with the stigma; and that
they may be seen advancing horizontally through the air towards the stigma. I
have observed the emission of the tubes from the pollen-masses whilst still
within the anthers, in three widely distinct Orchidean genera namely Aceras,
Malaxis, and Neottia: see 'The Various Contrivances by which Orchids are
Fertilised' 2nd edition page 258.) It is, however, a wonderful sight to behold
the tubes directing themselves in a straight line to the stigma, when this is at
some little distance from the anthers. As soon as they reach the stigma or the
open passage leading into the ovarium, no doubt they penetrate it, guided by the
same means, whatever these may be, as in the case of ordinary flowers. I thought
that they might be guided by the avoidance of light: some pollen-grains of a
willow were therefore immersed in an extremely weak solution of honey, and the
vessel was placed so that the light entered only in one direction, laterally or
from below or from above, but the long tubes were in each case protruded in
every possible direction.
As cleistogamic flowers are completely closed they are necessarily self-
fertilised, not to mention the absence of any attraction to insects; and they
thus differ widely from the great majority of ordinary flowers. Delpino believes
that cleistogamic flowers have been developed in order to ensure the production
of seeds under climatic or other conditions which tend to prevent the
fertilisation of the perfect flowers. (8/29. 'Sull' Opera la Distribuzione dei
Sessi nelle Piante' 1867 page 30.) I do not doubt that this holds good to a
certain limited extent, but the production of a large supply of seeds with
little consumption of nutrient matter or expenditure of vital force is probably
a far more efficient motive power. The whole flower is much reduced in size; but
what is much more important, an extremely small quantity of pollen has to be
formed, as none is lost through the action of insects or the weather; and pollen
contains much nitrogen and phosphorus. Von Mohl estimated that a single
cleistogamic anther-cell of Oxalis acetosella contained from one to two dozen
pollen-grains; we will say 20, and if so the whole flower can have produced at
most 400 grains; with Impatiens the whole number may be estimated in the same
manner at 250; with Leersia at 210; and with Viola nana at only 100. These
figures are wonderfully low compared with the 243,600 pollen-grains produced by
a flower of Leontodon, the 4,863 by an Hibiscus, or the 3,654,000 by a Paeony.
(8/30. The authorities for these statements are given in my 'Effects of Cross
and Self-Fertilisation' page 376.) We thus see that cleistogamic flowers produce
seeds with a wonderfully small expenditure of pollen; and they produce as a
general rule quite as many seeds as the perfect flowers.
That the production of a large number of seeds is necessary or beneficial to
many plants needs no evidence. So of course is their preservation before they
are ready for germination; and it is one of the many remarkable peculiarities of
the plants which bear cleistogamic flowers, that an incomparably larger
proportion of them than of ordinary plants bury their young ovaries in the
ground;--an action which it may be presumed serves to protect them from being
devoured by birds or other enemies. But this advantage is accompanied by the
loss of the power of wide dissemination. No less than eight of the genera in the
list at the beginning of this chapter include species which act in this manner,
namely, several kinds of Viola, Oxalis, Vandellia, Linaria, Commelina, and at
least three genera of Leguminosae. The seeds also of Leersia, though not buried,
are concealed in the most perfect manner within the sheaths of the leaves.
Cleistogamic flowers possess great facilities for burying their young ovaries or
capsules, owing to their small size, pointed shape, closed condition and the
absence of a corolla; and we can thus understand how it is that so many of them
have acquired this curious habit.
It has already been shown that in about 32 out of the 55 genera in the list just
referred to, the perfect flowers are irregular; and this implies that they have
been specially adapted for fertilisation by insects. Moreover three of the
genera with regular flowers are adapted by other means for the same end. Flowers
thus constructed are liable during certain seasons to be imperfectly fertilised,
namely, when the proper insects are scarce; and it is difficult to avoid the
belief that the production of cleistogamic flowers, which ensures under all
circumstances a full supply of seed, has been in part determined by the perfect
flowers being liable to fail in their fertilisation. But if this determining
cause be a real one, it must be of subordinate importance, as four of the genera
in the list are fertilised by the wind; and there seems no reason why their
perfect flowers should fail to be fertilised more frequently than those in any
other anemophilous genus. In contrast with what we here see with respect to the
large proportion of the perfect flowers being irregular, one genus alone out of
the 38 heterostyled genera described in the previous chapters bears such
flowers; yet all these genera are absolutely dependent on insects for their
legitimate fertilisation. I know not how to account for this difference in the
proportion of the plants bearing regular and irregular flowers in the two
classes, unless it be that the heterostyled flowers are already so well adapted
for cross-fertilisation, through the position of their stamens and pistils and
the difference in power of their two or three kinds of pollen, that any
additional adaptation, namely, through the flowers being made irregular, has
been rendered superfluous.
Although cleistogamic flowers never fail to yield a large number of seeds, yet
the plants bearing them usually produce perfect flowers, either simultaneously
or more commonly at a different period; and these are adapted for or admit of
cross-fertilisation. From the cases given of the two Indian species of Viola,
which produced in this country during several years only cleistogamic flowers,
and of the numerous plants of Vandellia and of some plants of Ononis which
behaved during one whole season in the same manner, it appears rash to infer
from such cases as that of Salvia cleistogama not having produced perfect
flowers during five years in Germany (8/31. Dr. Ascherson 'Botanische Zeitung'
1871 page 555.), and of an Aspicarpa not having done so during several years in
Paris, that these plants would not bear perfect flowers in their native homes.
Von Mohl and several other botanists have repeatedly insisted that as a general
rule the perfect flowers produced by cleistogamic plants are sterile; but it has
been shown under the head of the several species that this is not the case. The
perfect flowers Viola are indeed sterile unless they are visited by bees; but
when thus visited they yield the full number of seeds. As far as I have been
able to discover there is only one absolute exception to the rule that the
perfect flowers are fertile, namely, that of Voandzeia; and in this case we
should remember that cultivation often affects injuriously the reproductive
organs. Although the perfect flowers of Leersia sometimes yield seeds, yet this
occurs so rarely, as far as hitherto observed, that it practically forms a
second exception to the rule.
As cleistogamic flowers are invariably fertilised, and as they are produced in
large numbers, they yield altogether a much larger supply of seeds than do the
perfect flowers on the same plant. But the latter flowers will occasionally be
cross-fertilised, and their offspring will thus be invigorated, as we may infer
from a wide-spread analogy. But of such invigoration I have only a small amount
of direct evidence: two crossed seedlings of Ononis minutissima were put into
competition with two seedlings raised from cleistogamic flowers; they were at
first all of equal height; the crossed were then slightly beaten; but on the
following year they showed the usual superiority of their class, and were to the
self-fertilised plants of cleistogamic origin as 100 to 88 in mean height. With
Vandellia twenty crossed plants exceeded in height twenty plants raised from
cleistogamic seeds only by a little, namely, in the ratio of 100 to 94.
It is a natural inquiry how so many plants belonging to various very distinct
families first came to have the development of their flowers arrested, so as
ultimately to become cleistogamic. That a passage from the one state to the
other is far from difficult is shown by the many recorded cases of gradations
between the two states on the same plant, in Viola, Oxalis, Biophytum,
Campanula, etc. In the several species of Viola the various parts of the flowers
have also been modified in very different degrees. Those plants which in their
own country produce flowers of full or nearly full size, but never expand (as
with Thelymitra), and yet set fruit, might easily be rendered cleistogamic.
Lathyrus nissolia seems to be in an incipient transitional state, as does
Drosera Anglica, the flowers of which are not perfectly closed. There is good
evidence that flowers sometimes fail to expand and are somewhat reduced in size,
owing to exposure to unfavourable conditions, but still retain their fertility
unimpaired. Linnaeus observed in 1753 that the flowers on several plants brought
from Spain and grown at Upsala did not show any corolla and yet produced seeds.
Asa Gray has seen flowers on exotic plants in the Northern United States which
never expanded and yet fruited. With certain English plants, which bear flowers
during nearly the whole year, Mr. Bennett found that those produced during the
winter season were fertilised in the bud; whilst with other species having fixed
times for flowering, but "which had been tempted by a mild January to put forth
a few wretched flowers," no pollen was discharged from the anthers, and no seed
was formed. The flowers of Lysimachia vulgaris if fully exposed to the sun
expand properly, while those growing in shady ditches have smaller corollas
which open only slightly; and these two forms graduate into one another in
intermediate stations. Herr Bouche's observations are of especial interest, for
he shows that both temperature and the amount of light affect the size of the
corolla; and he gives measurements proving that with some plants the corolla is
diminished by the increasing cold and darkness of the changing season, whilst
with others it is diminished by the increasing heat and light. (8/32. For the
statement by Linnaeus see Mohl in 'Botanische Zeitung' 1863 page 327. Asa Gray
'American Journal of Science' 2nd series volume 39 1865 page 105. Bennett in
'Nature' November 1869 page 11. The Reverend G. Henslow also says 'Gardener's
Chronicle' 1877 page 271, also 'Nature' October 19, 1876 page 543, "that when
the autumn draws on, and habitually in winter for such of our wild flowers as
blossom at that season" the flowers are self-fertilised. On Lysimachia H. Muller
'Nature' September 1873 page 433. Bouche 'Sitzungsbericht der Gesell.
Naturforsch. Freunde' October 1874 page 90.)
The belief that the first step towards flowers being rendered cleistogamic was
due to the conditions to which they were exposed, is supported by the fact of
various plants belonging to this class either not producing their cleistogamic
flowers under certain conditions, or, on the other hand, producing them to the
complete exclusion of the perfect ones. Thus some species of Viola do not bear
cleistogamic flowers when growing on the lowlands or in certain districts. Other
plants when cultivated have failed to produce perfect flowers during several
successive years; and this is the case with Juncus bufonius in its native land
of Russia. Cleistogamic flowers are produced by some species late and by others
early in the season; and this agrees with the view that the first step towards
their development was due to climate; though the periods at which the two sorts
of flowers now appear must since have become much more distinctly defined. We do
not know whether too low are too high a temperature or the amount of light acts
in a direct manner on the size of the corolla, or indirectly through the male
organs being first affected. However this may be, if a plant were prevented
either early or late in the season from fully expanding its corolla, with some
reduction in its size, but with no loss of the power of self-fertilisation, then
natural selection might well complete the work and render it strictly
cleistogamic. The various organs would also, it is probable, be modified by the
peculiar conditions to which they are subjected within a completely closed
flower; also by the principle of correlated growth, and by the tendency in all
reduced organs finally to disappear. The result would be the production of
cleistogamic flowers such as we now see them; and these are admirably fitted to
yield a copious supply of seed at a wonderfully small cost to the plant.
I will now sum up very briefly the chief conclusions which seem to follow from
the observations given in this volume. Cleistogamic flowers afford, as just
stated, an abundant supply of seeds with little expenditure; and we can hardly
doubt that they have had their structure modified and degraded for this special
purpose; perfect flowers being still almost always produced so as to allow of
occasional cross-fertilisation. Hermaphrodite plants have often been rendered
monoecious, dioecious or polygamous; but as the separation of the sexes would
have been injurious, had not pollen been already transported habitually by
insects or by the wind from flower to flower, we may assume that the process of
separation did not commence and was not completed for the sake of the advantages
to be gained from cross-fertilisation. The sole motive for the separation of the
sexes which occurs to me, is that the production of a great number of seeds
might become superfluous to a plant under changed conditions of life; and it
might then be highly beneficial to it that the same flower or the same
individual should not have its vital powers taxed, under the struggle for life
to which all organisms are subjected, by producing both pollen and seeds. With
respect to the plants belonging to the gyno-dioecious sub-class, or those which
co-exist as hermaphrodites and females, it has been proved that they yield a
much larger supply of seed than they would have done if they had all remained
hermaphrodites; and we may feel sure from the large number of seeds produced by
many plants that such production is often necessary or advantageous. It is
therefore probable that the two forms in this sub-class have been separated or
developed for this special end.
Various hermaphrodite plants have become heterostyled, and now exist under two
or three forms; and we may confidently believe that this has been effected in
order that cross-fertilisation should be assured. For the full and legitimate
fertilisation of these plants pollen from the one form must be applied to the
stigma of another. If the sexual elements belonging to the same form are united
the union is an illegitimate one and more or less sterile. With dimorphic
species two illegitimate unions, and with trimorphic species twelve are
possible. There is reason to believe that the sterility of these unions has not
been specially acquired, but follows as an incidental result from the sexual
elements of the two or three forms having been adapted to act on one another in
a particular manner, so that any other kind of union is inefficient, like that
between distinct species. Another and still more remarkable incidental result is
that the seedlings from an illegitimate union are often dwarfed and more or less
or completely barren, like hybrids from the union of two widely distinct
Alefeld, Dr., on Linum.
Variability in length of stamens and pistil.
Anthers, size of, in different forms.
Ascherson, Dr., on Salvia cleistogama.
Ash, the common.
Axell on Primula stricta.
Babington, Professor, on Primula elatior.
Baillon, emission of the tubes from pollen-grains.
Belhomme, M., on ray-florets.
Bennett, A.W., on Impatiens fulva.
flowers fertilised whilst in the bud state.
Bentham, Mr., on the differentiation of the sexes.
on the cleistogamic flowers of Ononis.
Boreau on cowslip and primrose.
Bouche on Pavonia.
effect of temperature and light on corolla.
Braun on Dracocephalum.
Breitenbach, W., on Primula elatior.
Bromfield, Dr., on primrose and cowslip.
Brown, Robert, on sexual changes.
Buckwheat, the common.
Caspary, Professor, on Rhamnus catharticus.
list of genera.
on their origin.
Corolla, difference in size in the sexes of the same species.
Cowslip, the common.
short- and long-styled.
Crocker, C.W., on Plantago lanceolata.
Darwin, Charles, on reproductive organs under cultivation.
prepotency of pollen.
insects fertilising flowers.
Epidendron and Cattleya.
number of pollen-grains.
Darwin, W., on Pulmonaria angustifolia.
Delpino, plants fertilised by the wind.
on the walnut.
closed or cleistogamic flowers.
Dickie, Dr., on Eriophorum angustifolium.
Dioecious and sub-dioecious plants.
Doubleday, H., on Primula elatior.
Duval-Jouve, M., on Cryptostachys.
Dyer, Thiselton, on Salvia Horminum.
Fitzgerald, Mr., on Thelymitra.
Gartner on the sterility of unions between distinct species.
Primula vulgaris and veris.
prepotency of pollen.
variation in the sexual powers of plants.
Gillibert on Menyanthes.
Gloriosa Lily, the.
Godron on hybrid Primulas.
Gray, Professor Asa, proposes the term heterogone or heterogonous.
Leucosmia Burnettiana and acuminata.
Oxybaphus and Nyctaginia.
Hart, Mr., on Nepeta glechoma.
Hautbois Strawberry, the.
Henslow, Reverend Professor, on hybrid Primulae.
Henslow, Reverend G., on flowers self-fertilised during the winter.
Herbert, Dr., on hybrid Primulae.
Heterostyled plants, illegitimate offspring of.
essential character of.
summary of the differences of fertility between legitimately and illegitimately
diameter of pollen-grains.
size of anthers, structure of stigma.
list of genera.
advantages derived from Heterostylism.
means by which plants became heterostyled.
transmission of form.
-- dimorphic plants.
-- trimorphic plants.
Hildebrand, Professor, introduces the word "heterostyled."
on the ray-florets of the Compositae.
hermaphrodite plants becoming uni-sexual.
Homostyled species of Primula.
Hooker Dr., on Campanula.
papillae on stigma.
Illegitimate offspring of heterostyled plants.
Lythrum salicaria, dwarfed stature and sterility.
Oxalis, transmission of form to seedlings.
Primula Sinensis, in some degree dwarfed.
transmission of form and colour.
dwarfed stature and sterility.
parallelism between illegitimate fertilisation and hybridism.
Impatiens, pollen-grains of.
Jussieu, A. de, on Malpighiaceae.
Kerner, Professor, on ray-florets.
hybrid forms of Primula.
on use of hairs within the corolla.
size of corolla in male flowers.
use of glands as a protection to flowers.
Kirk, Dr., on Monochoria vaginalis.
Koch on Primula longiflora.
Kuhn, Dr., on cleistogamic flowers.
list of plants producing differently formed seeds.
Lecoq, H., on the common maple.
cowslips and primroses.
Leggett, Mr., Pontederia cordata.
Legitimate unions, summary on the fertility of the two, compared with that of
the two illegitimate in Primula.
fertility of, compared with illegitimate.
Leighton, Reverend W.A., on the cowslip and primrose.
Lily, the Gloriosa.
Lindley on Fragaria elatior.
Linnaeus on Primula veris, vulgaris, and elatior.
pistils and stamens.
sterile with its own-form pollen.
torsion of the styles.
power of mutual fertilisation between the three forms.
summary of results.
illegitimate offspring from the three forms.
concluding remarks on.
Maple, the common.
Marshall, W., on Primula elatior.
Masters, Dr. Maxwell, on cleistogamic flowers.
Maximowicz on Krascheninikowia.
Meehan, Mr., on Mitchella.
Mello, Correa de, on Arachis.
Michalet on Oxalis acetosella.
Mohl, H. Von, on the common cowslip.
size of corolla in the sexes of the same species.
Trifolium and Arachis.
Monnier, M., on Viola.
Muller, D., on Viola canina.
Muller, Fritz, on pollen of the Villarsia.
Muller, H., on the frequency of visits by insects to the Umbelliferae and
on Anthophora and Bombylius sucking the cowslip.
table of relative fertility of.
on the origin of heterostylism.
on the Labiatae.
size of corolla in the two sexes of the same species.
Nolana prostrata, variability in length of stamens and pistil.
Oliver, Professor, on ovules of Primula veris.
-- homostyled species.
-- (Biophytum) sensitiva.
Oxlip, the Bardfield.
--, the common.
differences in structure and function between the two parent-species.
effects of crossing.
a hybrid between the cowslip and primrose.
Paeony, pollen-grains of.
Parallelism between illegitimate and hybrid fertilisation.
Planchon on Linum salsoloides.
Pollen-grains, relative diameter of.
size of anthers.
Primrose, the common.
Primula, the, heterostyled species of.
-- equal-styled varieties.
-- elatior, Jacq.
relative fertility of the two forms.
not a hybrid.
equal-styled var. of.
transmission of form, constitution and fertility.
difference in structure between the two forms.
degrees of fertility when legitimately or illegitimately united.
fertility possessed by illegitimate plants.
equal-styled red variety.
length of pistil.
-- vulgaris (var. acaulis Linn.).
relative fertility of the two forms.
length of pistil
Primula vulgaris, var. rubra.
number of flowers.
Ray-florets, their use.
size of corolla.
size of anthers.
number of heterostyled genera.
Rue, the common.
Scott, J., on Primula auricula.
on Primula vulgaris.
on Primula var. rubra.
on Primula Sikkimensis.
on Primula farinosa.
length of pistil.
Eranthemum ambiguum bearing three kinds of flowers.
Smith, Sir J.E., on the carrot.
Spence, Mr., on Mollia.
Sprengel on Hottonia palustris.
Strawberry, the Hautbois.
Thomson, Dr., on Campanula.
Thrum-eyed, origin of term.
Thwaites, Mr., on ovules of Limnanthemum Indicum.
Timbal-Lagrave, M., on hybrids in genus Cistus.
Torrey, Dr., on Hottonia inflata.
Transmission of the two forms of heterostyled plants.
Treviranus on Androsace vitalliana.
Vaucher on the carrot.
Viola hirta and collina.
Verbascum, wild hybrids of.
Watson, H.C., on cowslips, primroses, and Oxlips.
Weddell, Dr., on hybrids between Aceras and Orchis.
Wetterhan, Mr., on Corylus.
Wichura, Max, on hybrid willows.
Wirtgen on Lythrum salicaria.
Wooler, W., on Polyanthus.
Wray, Leonard, on Fragaria.
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