The Different Forms of Flowers on Plants of the Same Species, Charles Robert Darwin [tohfa e dulha read online .TXT] 📗

forms, were protected under nets, and early in the season they produced spontaneously hardly any seeds, though at this period the artificially fertilised flowers produced an abundance; but it is a remarkable fact that later in the season, during September, both forms became highly self-fertile. They did not, however, produce so many seeds as some neighbouring uncovered plants which were visited by insects. Therefore the flowers of neither form when left to fertilise themselves late in the season without the aid of insects, are nearly so sterile as most other heterostyled plants. A large number of insects, namely 41 kinds as observed by H. Muller, visit the flowers for the sake of the eight drops of nectar. (3/14. 'Die Befruchtung' etc. page 175 and 'Nature' January 1, 1874 page 166.) He infers from the structure of the flowers that insects would be apt to fertilise them both illegitimately as well as legitimately; but he is mistaken in supposing that the long-styled flowers cannot spontaneously fertilise themselves.
Differently to what occurs in the other genera hitherto noticed, Polygonum, though a very large genus, contains, as far as is at present known, only a single heterostyled species, namely the present one. H. Muller in his interesting description of several other species shows that P. bistorta is so strongly proterandrous (the anthers generally falling off before the stigmas are mature) that the flowers must be cross-fertilised by the many insects which visit them. Other species bear much less conspicuous flowers which secrete little or no nectar, and consequently are rarely visited by insects; these are adapted for self-fertilisation, though still capable of cross-fertilisation. According to Delpino, the Polygonaceae are generally fertilised by the wind, instead of by insects as in the present genus.
[Leucosmia Burnettiana (Thymeliae).
As Professor Asa Gray has expressed his belief that this species and L. acuminata, as well as some species in the allied genus Drymispermum, are dimorphic or heterostyled (3/15. 'American Journal of Science' 1865 page 101 and Seemann's 'Journal of Botany' volume 3 1865 page 305.), I procured from Kew, through the kindness of Dr. Hooker, two dried flowers of the former species, an inhabitant of the Friendly Islands in the Pacific. The pistil of the long-styled form is to that of the short-styled as 100 to 86 in length; the stigma projects just above the throat of the corolla, and is surrounded by five anthers, the tips of which reach up almost to its base; and lower down, within the tubular corolla, five other and rather smaller anthers are seated. In the short-styled form, the stigma stands some way down the tube of the corolla, nearly on a level with the lower anthers of the other form: it differs remarkably from the stigma of the long-styled form, in being more papillose, and in being longer in the ratio of 100 to 60. The anthers of the upper stamens in the short-styled form are supported on free filaments, and project above the throat of the corolla, whilst the anthers of the lower stamens are seated in the throat on a level with the upper stamens of the other form. The diameters of a considerable number of grains from both sets of anthers in both forms were measured, but they did not differ in any trustworthy degree. The mean diameter of twenty-two grains from the short-styled flower was to that of twenty-four grains from the long-styled, as 100 to 99. The anthers of the upper stamens in the short-styled form appeared to be poorly developed, and contained a considerable number of shrivelled grains which were omitted in striking the above average. Notwithstanding the fact of the pollen-grains from the two forms not differing in diameter in any appreciable degree, there can hardly be a doubt from the great difference in the two forms in the length of the pistil, and especially of the stigma, together with its more papillose condition in the short-styled form, that the present species is truly heterostyled. This case resembles that of Linum grandiflorum, in which the sole difference between the two forms consists in the length of the pistils and stigmas. From the great length of the tubular corolla of Leucosmia, it is clear that the flowers are cross-fertilised by large Lepidoptera or by honey-sucking birds, and the position of the stamens in two whorls one beneath the other, which is a character that I have not seen in any other heterostyled dimorphic plant, probably serves to smear the inserted organ thoroughly with pollen.
Menyanthes trifoliata (Gentianeae).
This plant inhabits marshes: my son William gathered 247 flowers from so many distinct plants, and of these 110 were long-styled, and 137 short-styled. The pistil of the long-styled form is in length to that of the short-styled in the ratio of about 3 to 2. The stigma of the former, as my son observed, is decidedly larger than that of the short-styled; but in both forms it varies much in size. The stamens of the short-styled are almost double the length of those of the long-styled; so that their anthers stand rather above the level of the stigma of the long-styled form. The anthers also vary much in size, but seem often to be of larger size in the short-styled flowers. My son made with the camera many drawings of the pollen-grains, and those from the short-styled flowers were in diameter in nearly the ratio of 100 to 84 to those from the long-styled flowers. I know nothing about the capacity for fertilisation in the two forms; but short-styled plants, living by themselves in the gardens at Kew, have produced an abundance of capsules, yet the seeds have never germinated; and this looks as if the short-styled form was sterile with its own pollen.
Limnanthemum Indicum (Gentianeae).
This plant is mentioned by Mr. Thwaites in his Enumeration of the Plants of Ceylon as presenting two forms; and he was so kind as to send me specimens preserved in spirits. The pistil of the long-styled form is nearly thrice as long (i.e. as 14 to 5) as that of the short-styled, and is very much thinner in the ratio of about 3 to 5. The foliaceous stigma is more expanded, and twice as large as that of the short-styled form. In the latter the stamens are about twice as long as those of the long-styled, and their anthers are larger in the ratio of 100 to 70. The pollen-grains, after having been long kept in spirits, were of the same shape and size in both forms. The ovules, according to Mr. Thwaites, are equally numerous (namely from 70 to 80) in the two forms.
Villarsia [sp.?] (Gentianeae).
Fritz Muller sent me from South Brazil dried flowers of this aquatic plant, which is closely allied to Limnanthemum. In the long-styled form the stigma stands some way above the anthers, and the whole pistil, together with the ovary, is in length to that of the short-styled form as about 3 to 2. In the latter form the anthers stand above the stigma, and the style is very short and thick; but the pistil varies a good deal in length, the stigma being either on a level with the tips of the sepals or considerably beneath them. The foliaceous stigma in the long-styled form is larger, with the expansions running farther down the style, than in the other form. One of the most remarkable differences between the two forms is that the anthers of the longer stamens in the short- styled flowers are conspicuously longer than those of the shorter stamens in the long-styled flowers. In the former the sub-triangular pollen-grains are larger; the ratio between their breadth (measured from one angle to the middle of the opposite side) and that of the grains from the long-styled flowers being about 100 to 75. Fritz Muller also informs me that the pollen of the short-styled flowers has a bluish tint, whilst that of the long-styled is yellow. When we treat of Lythrum salicaria we shall find a strongly marked contrast in the colour of the pollen in two of the forms.
The three genera, Menyanthes, Limnanthemum, and Villarsia, now described, constitute a well-marked sub-tribe of the Gentianeae. All the species, as far as at present known, are heterostyled, and all inhabit aquatic or sub-aquatic stations.
Forsythia suspensa (Oleaceae).
Professor Asa Gray states that the plants of this species growing in the Botanic Gardens at Cambridge, U.S., are short-styled, but that Siebold and Zuccarini describe the long-styled form, and give figures of two forms; so that there can be little doubt, as he remarks, about the plant being dimorphic. (3/16. 'The American Naturalist' July 1873 page 422.) I therefore applied to Dr. Hooker, who sent me a dried flower from Japan, another from China, and another from the Botanic Gardens at Kew. The first proved to be long-styled, and the other two short-styled. In the long-styled form, the pistil is in length to that of the short-styled as 100 to 38, the lobes of the stigma being a little longer (as 10 to 9), but narrower and less divergent. This last character, however, may be only a temporary one. There seems to be no difference in the papillose condition of the two stigmas. In the short-styled form, the stamens are in length to those of the long-styled as 100 to 66, but the anthers are shorter in the ratio of 87 to 100; and this is unusual, for when there is any difference in size between the anthers of the two forms, those from the longer stamens of the short-styled are generally the longest. The pollen-grains from the short-styled flowers are certainly larger, but only in a slight degree, than those from the long-styled, namely, as 100 to 94 in diameter. The short-styled form, which grows in the Gardens at Kew, has never there produced fruit.
Forsythia viridissima appears likewise to be heterostyled; for Professor Asa Gray says that although the long-styled form alone grows in the gardens at Cambridge, U.S., the published figures of this species belong to the short- styled form.
Cordia [sp.?] (Cordiaceae).
Fritz Muller sent me dried specimens of this shrub, which he believes to be heterostyled; and I have not much doubt that this is the case, though the usual characteristic differences are not well pronounced in the two forms. Linum grandiflorum shows us that a plant may be heterostyled in function in the highest degree, and yet the two forms may have stamens of equal length, and pollen-grains of equal size. In the present species of Cordia, the stamens of both forms are of nearly equal length, those of the short-styled being rather the longest; and the anthers of both are seated in the mouth of the corolla. Nor could I detect any difference in the size of the pollen-grains, when dry or after being soaked in water. The stigmas of the long-styled form stand clear above the anthers, and the whole pistil is longer than that of the short-styled, in about the ratio of 3 to 2.
The stigmas of the short-styled form are seated beneath the anthers, and they are considerably shorter than those of the long-styled form. This latter difference is the most important one of any between the two forms.
Gilia (Ipomopsis) pulchella vel aggregata (Polemoniaceae).
Professor Asa Gray remarks with respect to this plant: "the tendency to dimorphism, of which there are traces, or perhaps rather incipient manifestations in various portions of the genus, is most marked in G. aggregata." (3/17. 'Proceedings of the American Academy of Arts and Sciences.' June 14, 1870 page 275.) He sent me some dried flowers, and I procured others from Kew. They differ greatly in size, some being nearly twice as long as others (namely as 30 to 17), so that it was not possible to compare, except by calculation, the absolute length of the organs from different plants. Moreover, the relative position of the stigmas and anthers is variable: in some long- styled