The Different Forms of Flowers on Plants of the Same Species by Charles Robert Darwin (rainbow fish read aloud txt) 📕

by the careful examination of many flowers, that this power has not as yet been perfected; and, consequently, that a certain proportion of the pollen is rendered useless, namely, that from the anthers which do not rotate properly. It thus appears that the development of the plant has not as yet been completed; the stamens have indeed acquired their proper length, but not their full and perfect power of rotation. (3/25. Fritz Muller gives another instance of the want of absolute perfection in the flowers of another member of the Rubiaceae, namely, Posoqueria fragrans, which is adapted in a most wonderful manner for cross-fertilisation by the agency of moths. (See 'Botanische Zeitung' 1866 Number 17.) In accordance with the nocturnal habits of these insects, most of the flowers open only during the night; but some open in the day, and the pollen of such flowers is robbed, as Fritz Muller has often seen, by humble-bees and other insects, without any benefit being thus conferred on the plant.)
The several points of difference in structure between the two forms of Faramea are highly remarkable. Until within a recent period, if any one had been shown two plants which differed in a uniform manner in the length of their stamens and pistils,--in the form of their stigmas,--in the manner of dehiscence and slightly in the size of their anthers,--and to an extraordinary degree in the diameter and structure of their pollen-grains, he would have declared it impossible that the two could have belonged to one and the same species.
[Suteria (species unnamed in the herbarium at Kew.) (Rubiaceae).
I owe to the kindness of Fritz Muller dried flowers of this plant from St. Catharina, in Brazil. In the long-styled form the stigma stands in the mouth of the corolla, above the anthers, which latter are enclosed within the tube, but only a short way down. In the short-styled form the anthers are placed in the mouth of the corolla above the stigma, which occupies the same position as the anthers in the other form, being seated only a short way down the tube. Therefore the pistil of the long-styled form does not exceed in length that of the short-styled in nearly so great a degree as in many other Rubiaceae. Nevertheless there is a considerable difference in the size of the pollen-grains in the two forms; for, as Fritz Muller informs me, those of the short-styled are to those of the long-styled as 100 to 75 in diameter.
Houstonia coerulea (Rubiaceae).
Professor Asa Gray has been so kind as to send me an abstract of some observations made by Dr. Rothrock on this plant. The pistil is exserted in the one form and the stamens in the other, as has long been observed. The stigmas of the long-styled form are shorter, stouter, and far more hispid than in the other form. The stigmatic hairs or papillae on the former are .04 millimetres, and on the latter only .023 millimetres in length. In the short-styled form the anthers are larger, and the pollen-grains, when distended with water, are to those from the long-styled form as 100 to 72 in diameter.
Selected capsules from some long-styled plants growing in the Botanic Gardens at Cambridge, U.S., near where plants of the other form grew, contained on an average 13 seeds; but these plants must have been subjected to unfavourable conditions, for some long-styled plants in a state of nature yielded an average of 21.5 seeds per capsule. Some short-styled plants, which had been planted by themselves in the Botanic Gardens, where it was not likely that they would have been visited by insects that had previously visited long-styled plants, produced capsules, eleven of which were wholly sterile, but one contained 4, and another 8 seeds. So that the short-styled form seems to be very sterile with its own pollen. Professor Asa Gray informs me that the other North American species of this genus are likewise heterostyled.
Oldenlandia [sp.?] (Rubiaceae).
Mr. J. Scott sent me from India dried flowers of a heterostyled species of this genus, which is closely allied to the last. The pistil in the long-styled flowers is longer by about a quarter of its length, and the stamens shorter in about the same proportion, than the corresponding organs in the short-styled flowers. In the latter the anthers are longer, and the divergent stigmas decidedly longer and apparently thinner than in the long-styled form. Owing to the state of the specimens, I could not decide whether the stigmatic papillae were longer in the one form than in the other. The pollen-grains, distended with water, from the short-styled flowers were to those from the long-styled as 100 to 78 in diameter, as deduced from the mean of ten measurements of each kind.
Hedyotis [sp.?] (Rubiaceae).
Fritz Muller sent me from St. Catharina, in Brazil, dried flowers of a small delicate species, which grows on wet sand near the edges of fresh-water pools. In the long-styled form the stigma projects above the corolla, and stands on a level with the projecting anthers of the short-styled form; but in the latter the stigmas stand rather beneath the level of the anthers in the other or long- styled form, these being enclosed within the tube of the corolla. The pistil of the long-styled form is nearly thrice as long as that of the short-styled, or, speaking strictly, as 100 to 39; and the papillae on the stigma of the former are broader, in the ratio of 4 to 3, but whether longer than those of the short- styled, I could not decide. In the short-styled form, the anthers are rather larger, and the pollen-grains are to those from the long-styled flowers, as 100 to 88 in diameter. Fritz Muller sent me a second, small-sized species, which is likewise heterostyled.
Coccocypselum [sp.?] (Rubiaceae).
Fritz Muller also sent me dried flowers of this plant from St. Catharina, in Brazil. The exserted stigma of the long-styled form stands a little above the level of the exserted anthers of the short-styled form; and the enclosed stigma of the latter also stands a little above the level of the enclosed anthers in the long-styled form. The pistil of the long-styled is about twice as long as that of the short-styled, with its two stigmas considerably longer, more divergent, and more curled. Fritz Muller informs me that he could detect no difference in the size of the pollen-grains in the two forms. Nevertheless, there can be no doubt that this plant is heterostyled.
Lipostoma [sp.?] (Rubiaceae).
Dried flowers of this plant, which grows in small wet ditches in St. Catharina, in Brazil, were likewise sent me by Fritz Muller. In the long-styled form the exserted stigma stands rather above the level of the exserted anthers of the other form; whilst in the short-styled form it stands on a level with the anthers of the other form. So that the want of strict correspondence in height between the stigmas and anthers in the two forms is reversed, compared with what occurs in Hedyotis. The long-styled pistil is to that of the short-styled as 100 to 36 in length; and its divergent stigmas are longer by fully one-third of their own length than those of the short-styled form. In the latter the anthers are a little larger, and the pollen-grains are as 100 to 80 in diameter, compared with those from the long-styled form.
Cinchona micrantha (Rubiaceae).
Dried specimens of both forms of this plant were sent me from Kew. (3/26. My attention was called to this plant by a drawing copied from Howard's 'Quinologia' Table 3 given by Mr. Markham in his 'Travels in Peru' page 539.) In the long-styled form the apex of the stigma stands just beneath the bases of the hairy lobes of the corolla; whilst the summits of the anthers are seated about halfway down the tube. The pistil is in length as 100 to 38 to that of the short-styled form. In the latter the anthers occupy the same position as the stigma of the other form, and they are considerably longer than those of the long-styled form. As the summit of the stigma in the short-styled form stands beneath the bases of the anthers, which are seated halfway down the corolla, the style has been extremely shortened in this form, its length to that of the long- styled being, in the specimens examined, only as 5.3 to 100! The stigma, also, in the short-styled form is very much shorter than that in the long-styled, in the ratio of 57 to 100. The pollen grains from the short-styled flowers, after having been soaked in water, were rather larger--in about the ratio of 100 to 91--than those from the long-styled flowers, and they were more triangular, with the angles more prominent. As all the grains from the short-styled flowers were thus characterised, and as they had been left in water for three days, I am convinced that this difference in shape in the two sets of grains cannot be accounted for by unequal distension with water.
Besides the several Rubiaceous genera already mentioned, Fritz Muller informs me that two or three species of Psychotria and Rudgea eriantha, natives of St. Catharina, in Brazil, are heterostyled, as is Manettia bicolor. I may add that I formerly fertilised with their own pollen several flowers on a plant of this latter species in my hothouse, but they did not set a single fruit. From Wight and Arnott's description, there seems to be little doubt that Knoxia in India is heterostyled; and Asa Gray is convinced that this is the case with Diodia and Spermacoce in the United States. Lastly, from Mr. W.W. Bailey's description, it appears that the Mexican Bouvardia leiantha is heterostyled. (3/27. 'Bulletin of the Torrey Bot. Club' 1876 page 106.)]
Altogether we now know of 17 heterostyled genera in the great family of the Rubiaceae; though more information is necessary with respect to some of them, more especially those mentioned in the last paragraph, before we can feel absolutely safe. In the 'Genera Plantarum,' by Bentham and Hooker, the Rubiaceae are divided into 25 tribes, containing 337 genera; and it deserves notice that the genera now known to be heterostyled are not grouped in one or two of these tribes, but are distributed in no less than eight of them. From this fact we may infer that most of the genera have acquired their heterostyled structure independently of one another; that is, they have not inherited this structure from some one or even two or three progenitors in common. It further deserves notice that in the homostyled genera, as I am informed by Professor Asa Gray, the stamens are either exserted or are included within the tube of the corolla, in a nearly constant manner; so that this character, which is not even of specific value in the heterostyled species, is often of generic value in other members of the family.


CHAPTER IV.
HETEROSTYLED TRIMORPHIC PLANTS.
Lythrum salicaria. Description of the three forms. Their power and complex manner of fertilising one another. Eighteen different unions possible. Mid-styled form eminently feminine in nature. Lythrum Graefferi likewise trimorphic. L. thymifolia dimorphic. L. Hyssopifolia homostyled. Nesaea verticillata trimorphic. Lagerstroemia, nature doubtful. Oxalis, trimorphic species of. O. Valdiviana. O. Regnelli, the illegitimate unions quite barren. O. speciosa. O. sensitiva. Homostyled species of Oxalis. Pontederia, the one monocotyledonous genus known to include heterostyled species.
In the previous chapters various heterostyled dimorphic plants have been described, and now we come to heterostyled trimorphic plants, or those which present three forms. These have been observed in three families, and consist of species of Lythrum and of the allied genus Nesaea, of Oxalis and Pontederia. In their manner of fertilisation these plants offer a more remarkable case than can be found in any other plant or animal.
Lythrum salicaria.
The pistil in each form differs from that in either of the other forms, and in each there