The Different Forms of Flowers on Plants of the Same Species - Charles Robert Darwin (ebook reader screen TXT) 📗

and inconspicuous sepals of the female have been reduced in size; and in the strawberry the flowers are largest in the males, mid-sized in the hermaphrodites, and smallest in the females. These latter cases,--the variability in the size of the corolla in some of the above species, for instance in the common thyme,--together with the fact that it never differs greatly in size in the two forms--make me doubt much whether natural selection has come into play;--that is whether, in accordance with H. Muller's belief, the advantage derived from the polleniferous flowers being visited first by insects has been sufficient to lead to a gradual reduction of the corolla of the female. We should bear in mind that as the hermaphrodite is the normal form, its corolla has probably retained its original size. (7/24. It does not appear to me that Kerner's view 'Die Schutzmittel des Pollens' 1873 page 56, can be accepted in the present cases, namely that the larger corolla in the hermaphrodites and males serves to protect their pollen from rain. In the genus Thymus, for instance, the aborted anthers of the female are much better protected than the perfect ones of the hermaphrodite.) An objection to the above view should not be passed over; namely, that the abortion of the stamens in the females ought to have added through the law of compensation to the size of the corolla; and this perhaps would have occurred, had not the expenditure saved by the abortion of the stamens been directed to the female reproductive organs, so as to give to this form increased fertility.


CHAPTER VIII. CLEISTOGAMIC FLOWERS.
General character of cleistogamic flowers. List of the genera producing such flowers, and their distribution in the vegetable series. Viola, description of the cleistogamic flowers in the several species; their fertility compared with that of the perfect flowers. Oxalis acetosella. O. sensitiva, three forms of cleistogamic flowers. Vandellia. Ononis. Impatiens. Drosera. Miscellaneous observations on various other cleistogamic plants. Anemophilous species producing cleistogamic flowers. Leersia, perfect flowers rarely developed. Summary and concluding remarks on the origin of cleistogamic flowers. The chief conclusions which may be drawn from the observations in this volume.
It was known even before the time of Linnaeus that certain plants produced two kinds of flowers, ordinary open, and minute closed ones; and this fact formerly gave rise to warm controversies about the sexuality of plants. These closed flowers have been appropriately named cleistogamic by Dr. Kuhn. (8/1. 'Botanische Zeitung' 1867 page 65.) They are remarkable from their small size and from never opening, so that they resemble buds; their petals are rudimentary or quite aborted; their stamens are often reduced in number, with the anthers of very small size, containing few pollen-grains, which have remarkably thin transparent coats, and generally emit their tubes whilst still enclosed within the anther-cells; and, lastly, the pistil is much reduced in size, with the stigma in some cases hardly at all developed. These flowers do not secrete nectar or emit any odour; from their small size, as well as from the corolla being rudimentary, they are singularly inconspicuous. Consequently insects do not visit them; nor if they did, could they find an entrance. Such flowers are therefore invariably self-fertilised; yet they produce an abundance of seed. In several cases the young capsules bury themselves beneath the ground, and the seeds are there matured. These flowers are developed before, or after, or simultaneously with the perfect ones. Their development seems to be largely governed by the conditions to which the plants are exposed, for during certain seasons or in certain localities only cleistogamic or only perfect flowers are produced.
Dr. Kuhn, in the article above referred to, gives a list of 44 genera including species which bear flowers of this kind. To this list I have added some genera, and the authorities are appended in a footnote. I have omitted three names, from reasons likewise given in the footnote. But it is by no means easy to decide in all cases whether certain flowers ought to be ranked as cleistogamic. For instance, Mr. Bentham informs me that in the South of France some of the flowers on the vine do not fully open and yet set fruit; and I hear from two experienced gardeners that this is the case with the vine in our hothouses; but as the flowers do not appear to be completely closed it would be imprudent to consider them as cleistogamic. The flowers of some aquatic and marsh plants, for instance of Ranunculus aquatalis, Alisma natans, Subularia, Illecebrum, Menyanthes, and Euryale, remain closely shut as long as they are submerged, and in this condition fertilise themselves. (8/2. Delpino 'Sull' Opera, la Distribuzione dei Sessi nelle Piante' etc. 1867 page 30. Subularia, however, sometimes has its flowers fully expanded beneath the water, see Sir J.E. Smith 'English Flora' volume 3 1825 page 157. For the behaviour of Menyanthes in Russia see Gillibert in 'Act. Acad. St. Petersb.' 1777 part 2 page 45.--On Euryale 'Gardener's Chronicle' 1877 page 280.) They behave in this manner, apparently as a protection to their pollen, and produce open flowers when exposed to the air; so that these cases seem rather different from those of true cleistogamic flowers, and have not been included in the list. Again, the flowers of some plants which are produced very early or very late in the season do not properly expand; and these might perhaps be considered as incipiently cleistogamic; but as they do not present any of the remarkable peculiarities proper to the class, and as I have not found any full record of such cases, they are not entered in the list. When, however, it is believed on fairly good evidence that the flowers on a plant in its native country do not open at any hour of the day or night, and yet set seeds capable of germination, these may fairly be considered as cleistogamic, notwithstanding that they present no peculiarities of structure. I will now give as complete a list of the genera containing cleistogamic species as I have been able to collect.
TABLE 8.38. List of genera including cleistogamic species (chiefly after Kuhn). (8/3. I have omitted Trifolium and Arachis from the list, because Von Mohl says 'Botanische Zeitung' 1863 page 312, that the flower-stems merely draw the flowers beneath the ground, and that these do not appear to be properly cleistogamic. Correa de Mello 'Journal of the Linnean Society Botany' volume 11 1870 page 254, observed plants of Arachis in Brazil, and could never find such flowers. Plantago has been omitted because as far as I can discover it produces hermaphrodite and female flower-heads, but not cleistogamic flowers. Krascheninikowia (vel Stellaria) has been omitted because it seems very doubtful from Maximowicz' description whether the lower flowers which have no petals or very small ones, and barren stamens or none, are cleistogamic; the upper hermaphrodite flowers are said never to produce fruit, and therefore probably act as males. Moreover in Stellaria graminea, as Babington remarks 'British Botany' 1851 page 51, "shorter and longer petals accompany an imperfection of the stamens or germen."
I have added to the list the following cases: Several Acanthaceae, for which see J. Scott in 'Journal of Botany' London new series volume 1 1872 page 161. With respect to salvia see Dr. Ascherson in 'Botanische Zeitung' 1871 page 555. For Oxybaphus and Nyctaginia see Asa Gray in 'American Naturalist' November 1873 page 692. From Dr. Torrey's account of Hottonia inflata 'Bulletin of the Torrey Botanical Club' volume 2 June 1871, it is manifest that this plant produces true cleistogamic flowers. For Pavonia see Bouche in 'Sitzungsberichte d. Gesellsch. Natur. Freunde' October 20, 1874 page 90. I have added Thelymitra, as from the account given by Mr. Fitzgerald in his magnificent work on 'Australian Orchids' it appears that the flowers of this plant in its native home never open, but they do not appear to be reduced in size. Nor is this the case with the flowers of certain species of Epidendron, Cattleya, etc. see second edition of my 'Fertilisation of Orchids' page 147, which without expanding produce capsules. It is therefore doubtful whether these Orchideae ought to have been included in the list. From what Duval-Jouve says about Cryptostachys in 'Bulletin of the Soc. Bot. de France' tome 10 1863 page 195, this plant appears to produce cleistogamic flowers. the other additions to the list are noticed in my text.)
DICOTYLEDONS.
BORAGINEAE: Eritrichium.
CONVOLVULACEAE: Cuscuta.
SCROPHULARINEAE: Scrophularia. Linaria. Vandellia.
ACANTHACEAE: Cryphiacanthus. Eranthemum. Daedalacanthus. Dipteracanthus. Aechmanthera. Ruellia.
LABIATAE: Lamium. Salvia.
NYCTAGINEAE: Oxybaphus. Nyctaginia.
ASCLEPIADAE: Stapelia.
CAMPANULACEAE: Specularia. Campanula.
PRIMULACEAE: Hottonia.
COMPOSITAE: Anandria.
CRUCIFERAE: Heterocarpaea.
VIOLACEAE: Viola.
CISTINEAE: Helianthemum. Lechea.
MALVEACEAE: Pavonia.
MALPIGHIACEAE: Gaudichaudia. Aspicarpa. Camarea. Janusia.
POLYGALEAE: Polygala.
BALSAMINEAE: Impatiens.
GERANIACAEA: Oxalis.
LEGUMINOSAE: Ononis. Parochaetus. Chapmannia. Stylosanthus. Lespedeza. Vicia. Lathyrus. Martinsia vel Neurocarpum. Amphicarpaea. Glycine. Galactia. Voandzeia.
DROSERACEAE: Drosera.
MONOCOTYLEDONS.
JUNCEAE: Juncus.
GRAMINEAE: Leersia. Hordeum. Cryptostachys.
COMMELINEAE: Commelina.
PONTEDERACEAE: Monochoria.
ORCHIDEAE: Schomburgkia. Cattleya. Epidendron. Thelymitra.
The first point that strikes us in considering this list of 55 genera, is that they are very widely distributed in the vegetable series. They are more common in the family of the Leguminosae than in any other, and next in order in that of the Acanthaceae and Malpighiaceae. A large number, but not all the species, of certain genera, as of Oxalis and Viola, bear cleistogamic as well as ordinary flowers. A second point which deserves notice is that a considerable proportion of the genera produce more or less irregular flowers; this is the case with about 32 out of the 55 genera, but to this subject I shall recur.
I formerly made many observations on cleistogamic flowers, but only a few of them are worth giving, since the appearance of an admirable paper by Hugo Von Mohl, whose examination was in some respects much more complete than mine. (8/4. 'Botanische Zeitung' 1863 page 309-28.) His paper includes also an interesting history of our knowledge on the subject.
Viola canina.
The calyx of the cleistogamic flowers differs in no respect from that of the perfect ones. The petals are reduced to five minute scales; the lower one, which represents the lower lip, is considerably larger than the others, but with no trace of the spur-like nectary; its margins are smooth, whilst those of the other four scale-like petals are papillose. D. Muller of Upsala says that in the specimens which he observed the petals were completely aborted. (8/5. Ibid. 1857 page 730. This paper contains the first full and satisfactory account of any cleistogamic flower.) The stamens are very small, and only the two lower ones are provided with anthers, which do not cohere together as in the perfect flowers. The anthers are minute, with the two cells or loculi remarkably distinct; they contain very little pollen in comparison with those of the perfect flowers. The connective expands into a membranous hood-like shield which projects above the anther-cells. These two lower stamens have no vestige of the curious appendages which secrete nectar in the perfect flowers. The three other stamens are destitute of anthers and have broader filaments, with their terminal membranous expansions flatter or not so hood-like as those of the two antheriferous stamens. The pollen-grains have remarkably thin transparent coats; when exposed to the air they shrivel up quickly; when placed in water they swell, and are then 8-10/7000 of an inch in diameter, and therefore of smaller size than the ordinary pollen-grains similarly treated, which have a diameter of 13-14/7000 of an inch. In the cleistogamic flowers, the pollen-grains, as far as I could see, never naturally fall out of the anther-cells, but emit their tubes through a pore at the upper end. I was able to trace the tubes from the grains some way down the stigma. The pistil is very short, with the style hooked, so that its extremity, which is a little enlarged or funnel-shaped and represents the stigma, is directed downwards, being covered by the two