The Different Forms of Flowers on Plants of the Same Species Part 28

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 dest.i.tute 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 membranous expansions of the antheriferous stamens. It is remarkable that there is an open pa.s.sage from the enlarged funnel-shaped extremity to within the ovarium; this was evident, as slight pressure caused a bubble of air, which had been drawn in by some accident, to travel freely from one end to the other: a similar pa.s.sage was observed by Michalet in V. alba. The pistil therefore differs considerably from that of the perfect flower; for in the latter it is much longer, and straight with the exception of the rectangularly bent stigma; nor is it perforated by an open pa.s.sage.

The ordinary or perfect flowers have been said by some authors never to produce capsules; but this is an error, though only a small proportion of them do so.

This appears to depend in some cases on their anthers not containing even a trace of pollen, but more generally on bees not visiting the flowers. I twice covered with a net a group of flowers, and marked with threads twelve of them which had not as yet expanded. This precaution is necessary, for though as a general rule the perfect flowers appear considerably before the cleistogamic ones, yet occasionally some of the latter are produced early in the season, and their capsules might readily be mistaken for those produced by the perfect flowers. Not one of the twelve marked perfect flowers yielded a capsule, whilst others under the net which had been artificially fertilised produced five capsules; and these contained exactly the same average number of seeds as some capsules from flowers outside the net which had been fertilised by bees. I have repeatedly seen Bombus hortorum, lapidarius, and a third species, as well as hive-bees, sucking the flowers of this violet: I marked six which were thus visited, and four of them produced fine capsules; the two others were gnawed off by some animal. I watched Bombus hortorum for some time, and whenever it came to a flower which did not stand in a convenient position to be sucked, it bit a hole through the spur-like nectary. Such ill-placed flowers would not yield any seed or leave descendants; and the plants bearing them would thus tend to be eliminated through natural selection.

The seeds produced by the cleistogamic and perfect flowers do not differ in appearance or number. On two occasions I fertilised several perfect flowers with pollen from other individuals, and afterwards marked some cleistogamic flowers on the same plants; and the result was that 14 capsules produced by the perfect flowers contained on an average 9.85 seeds; and 17 capsules from the cleistogamic ones contained 9.64 seeds,--an amount of difference of no significance. It is remarkable how much more quickly the capsules from the cleistogamic flowers are developed than those from the perfect ones; for instance, several perfect flowers were cross-fertilised on April 14th, 1863, and a month afterwards (May 15th) eight young cleistogamic flowers were marked with threads; and when the two sets of capsules thus produced were compared on June 3rd, there was scarcely any difference between them in size.

Viola odorata (WHITE-FLOWERED, SINGLE, CULTIVATED VARIETY).

The petals are represented by mere scales as in the last species; but differently from in the last, all five stamens are provided with diminutive anthers. Small bundles of pollen-tubes were traced from the five anthers into the somewhat distant stigma. The capsules produced by these flowers bury themselves in the soil, if it be loose enough, and there mature themselves.

(8/6. Vaucher says 'Hist. Phys. des Plantes d'Europe' tome 3 1844 page 309, that V. hirta and collina likewise bury their capsules. See also Lecoq 'Geograph.

Bot.' tome 5 1856 page 180.) Lecoq says that it is only these latter capsules which possess elastic valves; but I think this must be a misprint, as such valves would obviously be of no use to the buried capsules, but would serve to scatter the seeds of the sub-aerial ones, as in the other species of Viola. It is remarkable that this plant, according to Delpino, does not produce cleistogamic flowers in one part of Liguria, whilst the perfect flowers are there abundantly fertile (8/7. 'Sull' Opera, la Distribuzione dei Sessi nelle Piante' etc. 1867 page 30.); on the other hand, cleistogamic flowers are produced by it near Turin. Another fact is worth giving as an instance of correlated development: I found on a purple variety, after it had produced its perfect double flowers, and whilst the white single variety was bearing its cleistogamic flowers, many bud-like bodies which from their position on the plant were certainly of a cleistogamic nature. They consisted, as could be seen on bisecting them, of a dense ma.s.s of minute scales closely folded over one another, exactly like a cabbage-head in miniature. I could not detect any stamens, and in the place of the ovarium there was a little central column. The doubleness of the perfect flowers had thus spread to the cleistogamic ones, which therefore were rendered quite sterile.

Viola hirta.

The five stamens of the cleistogamic flowers are provided, as in the last case, with small anthers, from all of which pollen-tubes proceed to the stigma. The petals are not quite so much reduced as in V. canina, and the short pistil instead of being hooked is merely bent into a rectangle. Of several perfect flowers which I saw visited by hive-and humble-bees, six were marked, but they produced only two capsules, some of the others having been accidentally injured.

M. Monnier was therefore mistaken in this case as in that of V. odorata, in supposing that the perfect flowers always withered away and aborted. He states that the peduncles of the cleistogamic flowers curve downwards and bury the ovaries beneath the soil. (8/8. These statements are taken from Professor Oliver's excellent article in the 'Natural History Review' July 1862 page 238.

With respect to the supposed sterility of the perfect flowers in this genus see also Timbal-Lagrave 'Botanische Zeitung' 1854 page 772.) I may here add that Fritz Muller, as I hear from his brother, has found in the highlands of Southern Brazil a white-flowered species of violet which bears subterranean cleistogamic flowers.

Viola nana.

Mr. Scott sent me seeds of this Indian species from the Sikkim Terai, from which I raised many plants, and from these other seedlings during several successive generations. They produced an abundance of cleistogamic flowers during the whole of each summer, but never a perfect one. When Mr. Scott wrote to me his plants in Calcutta were behaving similarly, though his collector saw the species in flower in its native site. This case is valuable as showing that we ought not to infer, as has sometimes been done, that a species does not bear perfect flowers when growing naturally, because it produces only cleistogamic flowers under culture. The calyx of these flowers is sometimes formed of only three sepals; two being actually suppressed and not merely coherent with the others; this occurred with five out of thirty flowers which were examined for this purpose.

The petals are represented by extremely minute scales. Of the stamens, two bear anthers which are in the same state as in the previous species, but, as far as I could judge, each of the two cells contained only from 20 to 25 delicate transparent pollen-grains. These emitted their tubes in the usual manner. The three other stamens bore very minute rudimentary anthers, one of which was generally larger than the other two, but none of them contained any pollen. In one instance, however, a single cell of the larger rudimentary anther included a little pollen. The style consists of a short flattened tube, somewhat expanded at its upper end, and this forms an open channel leading into the ovarium, as described under V. canina. It is slightly bent towards the two fertile anthers.

Viola Roxburghiana.

This species bore in my hothouse during two years a mult.i.tude of cleistogamic flowers, which resembled in all respects those of the last species; but no perfect ones were produced. Mr. Scott informs me that in India it bears perfect flowers only during the cold season, and that these are quite fertile. During the hot, and more especially during the rainy season, it bears an abundance of cleistogamic flowers.

Many other species, besides the five now described, produce cleistogamic flowers; this is the case, according to D. Muller, Michalet, Von Mohl, and Hermann Muller, with V. elatior, lancifolia, sylvatica, pal.u.s.tris, mirabilis, bicolor, ionodium, and biflora. But V. tricolor does not produce them.

Michalet a.s.serts that V. pal.u.s.tris produces near Paris only perfect flowers, which are quite fertile; but that when the plant grows on mountains cleistogamic flowers are produced; and so it is with V. biflora. The same author states that he has seen in the case of V. alba flowers intermediate in structure between the perfect and cleistogamic ones. According to M. Boisduval, an Italian species, V.

Ruppii, never bears in France "des fleurs bien apparentes, ce qui ne l'empeche pas de fructifier."

It is interesting to observe the gradation in the abortion of the parts in the cleistogamic flowers of the several foregoing species. It appears from the statements by D. Muller and Von Mohl that in V. mirabilis the calyx does not remain quite closed; all five stamens are provided with anthers, and some pollen-grains probably fall out of the cells on the stigma, instead of protruding their tubes whilst still enclosed, as in the other species. In V.

hirta all five stamens are likewise antheriferous; the petals are not so much reduced and the pistil not so much modified as in the following species. In V.

nana and elatior only two of the stamens properly bear anthers, but sometimes one or even two of the others are thus provided. Lastly, in V. canina never more than two of the stamens, as far as I have seen, bear anthers; the petals are much more reduced than in V. hirta, and according to D. Muller are sometimes quite absent.

Oxalis acetosella.

The existence of cleistogamic flowers on this plant was discovered by Michalet.

(8/9. 'Bulletin Soc. Bot. de France' tome 7 1860 page 465.) They have been fully described by Von Mohl, and I can add hardly anything to his description. In my specimens the anthers of the five longer stamens were nearly on a level with the stigmas; whilst the smaller and less plainly bilobed anthers of the five shorter stamens stood considerably below the stigmas, so that their tubes had to travel some way upwards. According to Michalet these latter anthers are sometimes quite aborted. In one case the tubes, which ended in excessively fine points, were seen by me stretching upwards from the lower anthers towards the stigmas, which they had not as yet reached. My plants grew in pots, and long after the perfect flowers had withered they produced not only cleistogamic but a few minute open flowers, which were in an intermediate condition between the two kinds. In one of these the pollen-tubes from the lower anthers had reached the stigmas, though the flower was open. The footstalks of the cleistogamic flowers are much shorter than those of the perfect flowers, and are so much bowed downwards that they tend, according to Von Mohl, to bury themselves in the moss and dead leaves on the ground. Michalet also says that they are often hypogean. In order to ascertain the number of seeds produced by these flowers, I marked eight of them; two failed, one cast its seed abroad, and the remaining five contained on an average 10.0 seeds per capsule. This is rather above the average 9.2, which eleven capsules from perfect flowers fertilised with their own pollen yielded, and considerably above the average 7.9, from the capsules of perfect flowers fertilised with pollen from another plant; but this latter result must, I think, have been accidental.

Hildebrand, whilst searching various Herbaria, observed that many other species of Oxalis besides O. acetosella produce cleistogamic flowers (8/10.

'Monatsbericht der Akad. der Wiss. zu Berlin' 1866 page 369.); and I hear from him that this is the case with the heterostyled trimorphic O. incarnata from the Cape of Good Hope.

Oxalis (Biophytum) sensitiva.

This plant is ranked by many botanists as a distinct genus, but as a sub-genus by Bentham and Hooker. Many of the early flowers on a mid-styled plant in my hothouse did not open properly, and were in an intermediate condition between cleistogamic and perfect. Their petals varied from a rudiment to about half their proper size; nevertheless they produced capsules. I attributed their state to unfavourable conditions, for later in the season fully expanded flowers of the proper size appeared. But Mr. Thwaites afterwards sent me from Ceylon a number of long-styled, mid-styled, and short-styled flower-stalks preserved in spirits; and on the same stalks with the perfect flowers, some of which were fully expanded and others still in bud, there were small bud-like bodies containing mature pollen, but with their calyces closed. These cleistogamic flowers do not differ much in structure from the perfect ones of the corresponding form, with the exception that their petals are reduced to extremely minute, barely visible scales, which adhere firmly to the rounded bases of the shorter stamens. Their stigmas are much less papillose, and smaller in about the ratio of 13 to 20 divisions of the micrometer, as measured transversely from apex to apex, than the stigmas of the perfect flowers. The styles are furrowed longitudinally, and are clothed with simple as well as glandular hairs, but only in the cleistogamic flowers produced by the long- styled and mid-styled forms. The anthers of the longer stamens are a little smaller than the corresponding ones of the perfect flowers, in about the ratio of 11 to 14. They dehisce properly, but do not appear to contain much pollen.

Many pollen-grains were attached by short tubes to the stigmas; but many others, still adhering to the anthers, had emitted their tubes to a considerable length, without having come in contact with the stigmas. Living plants ought to be examined, as the stigmas, at least of the long-styled form, project beyond the calyx, and if visited by insects (which, however, is very improbable) might be fertilised with pollen from a perfect flower. The most singular fact about the present species is that long-styled cleistogamic flowers are produced by the long-styled plants, and mid-styled as well as short-styled cleistogamic flowers by the other two forms; so that there are three kinds of cleistogamic and three kinds of perfect flowers produced by this one species! Most of the heterostyled species of Oxalis are more or less sterile, many absolutely so, if illegitimately fertilised with their own-form pollen. It is therefore probable that the pollen of the cleistogamic flowers has been modified in power, so as to act on their own stigmas, for they yield an abundance of seeds. We may perhaps account for the cleistogamic flowers consisting of the three forms, through the principle of correlated growth, by which the cleistogamic flowers of the double violet have been rendered double.

Vandellia nummularifolia.

Dr. Kuhn has collected all the notices with respect to cleistogamic flowers in this genus, and has described from dried specimens those produced by an Abyssinian species. (8/11. 'Botanische Zeitung' 1867 page 65.) Mr. Scott sent me from Calcutta seeds of the above common Indian weed, from which many plants were successively raised during several years. The cleistogamic flowers are very small, being when fully mature under 1/20 of an inch (1.27 millimetres) in length. The calyx does not open, and within it the delicate transparent corolla remains closely folded over the ovarium. There are only two anthers instead of the normal number of four, and their filaments adhere to the corolla. The cells of the anthers diverge much at their lower ends and are only 5/700 of an inch (.181 millimetres) in their longer diameter. They contain but few pollen-grains, and these emit their tubes whilst still within the anther. The pistil is very short, and is surmounted by a bilobed stigma. As the ovary grows the two anthers together with the shrivelled corolla, all attached by the dried pollen-tubes to the stigma, are torn off and carried upwards in the shape of a little cap. The perfect flowers generally appear before the cleistogamic, but sometimes simultaneously with them. During one season a large number of plants produced no perfect flowers. It has been a.s.serted that the latter never yield capsules; but this is a mistake, as they do so even when insects are excluded. Fifteen capsules from cleistogamic flowers on plants growing under favourable conditions contained on an average 64.2 seeds, with a maximum of 87; whilst 20 capsules from plants growing much crowded yielded an average of only 48. Sixteen capsules from perfect flowers artificially crossed with pollen from another plant contained on an average 93 seeds, with a maximum of 137. Thirteen capsules from self-fertilised perfect flowers gave an average of 62 seeds, with a maximum of 135. Therefore the capsules from the cleistogamic flowers contained fewer seeds than those from perfect flowers when cross-fertilised, and slightly more than those from perfect flowers self-fertilised.

Dr. Kuhn believes that the Abyssinian V. sessiflora does not differ specifically from the foregoing species. But its cleistogamic flowers apparently include four anthers instead of two as above described. The plants, moreover, of V.

sessiflora produce subterranean runners which yield capsules; and I never saw a trace of such runners in V. nummularifolia, although many plants were cultivated.

Linaria spuria.

Michalet says that short, thin, twisted branches are developed from the buds in the axils of the lower leaves, and that these bury themselves in the ground.

(8/12. 'Bulletin Soc. Bot. de France' tome 7 1860 page 468.) They there produce flowers not offering any peculiarity in structure, excepting that their corollas, though properly coloured, are deformed. These flowers may be ranked as cleistogamic, as they are developed, and not merely drawn, beneath the ground.

Ononis columnae.

Plants were raised from seeds sent me from Northern Italy. The sepals of the cleistogamic flowers are elongated and closely pressed together; the petals are much reduced in size, colourless, and folded over the interior organs. The filaments of the ten stamens are united into a tube, and this is not the case, according to Von Mohl, with the cleistogamic flowers of other Leguminosae. Five of the stamens are dest.i.tute of anthers, and alternate with the five thus provided. The two cells of the anthers are minute, rounded and separated from one another by connective tissue; they contain but few pollen-grains, and these have extremely delicate coats. The pistil is hook-shaped, with a plainly enlarged stigma, which is curled down, towards the anthers; it therefore differs much from that of the perfect flower. During the year 1867 no perfect flowers were produced, but in the following year there were both perfect and cleistogamic ones.

Ononis minutissima.