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

manifestly differed greatly in size. Those from the short-styled flowers were to those from the long-styled in diameter in about the ratio of 100 to 62. The two forms of Ae. mollis present a like difference in the length of their pistils and stamens.
Aegiphila obdurata.
Flowers of this bush were sent me from St. Catharina in Brazil, by Fritz Muller, and were named for me at Kew. They appeared at first sight grandly heterostyled, as the stigma of the long-styled form projects far out of the corolla, whilst the anthers are seated halfway down within the tube; whereas in the short-styled form the anthers project from the corolla and the stigma is enclosed in the tube at nearly the same level with the anthers of the other form. The pistil of the long-styled is to that of the short-styled as 100 to 60 in length, and the stigmas, taken by themselves, as 100 to 55. Nevertheless, this plant cannot be heterostyled. The anthers in the long-styled form are brown, tough, and fleshy, and less than half the length of those in the short-styled form, strictly as 44 to 100; and what is much more important, they were in a rudimentary condition in the two flowers examined by me, and did not contain a single grain of pollen. In the short-styled form, the divided stigma, which as we have seen is much shortened, is thicker and more fleshy than the stigma of the long-styled, and is covered with small irregular projections, formed of rather large cells. It had the appearance of having suffered from hyperthrophy, and is probably incapable of fertilisation. If this be so the plant is dioecious, and judging from the two species previously described, it probably was once heterostyled, and has since been rendered dioecious by the pistil in the one form, and the stamens in the other having become functionless and reduced in size. It is, however, possible that the flowers may be in the same state as those of the common thyme and of several other Labiatae, in which females and hermaphrodites regularly co-exist. Fritz Muller, who thought that the present plant was heterostyled, as I did at first, informs me that he found bushes in several places growing quite isolated, and that these were completely sterile; whilst two plants growing close together were covered with fruit. This fact agrees better with the belief that the species is dioecious than that it consists of hermaphrodites and females; for if any one of the isolated plants had been an hermaphrodite, it would probably have produced some fruit.]


RUBIACEAE.
This great natural family contains a much larger number of heterostyled genera than any other one, as yet known.
Mitchella repens.
Professor Asa Gray sent me several living plants collected when out of flower, and nearly half of these proved long-styled, and the other half short-styled. The white flowers, which are fragrant and which secrete plenty of nectar, always grow in pairs with their ovaries united, so that the two together produce "a berry-like double drupe." (3/22. A. Gray 'Manual of the Botany of the United States' 1856 page 172.) In my first series of experiments (1864) I did not suppose that this curious arrangement of the flowers would have any influence on their fertility; and in several instances only one of the two flowers in a pair was fertilised; and a large proportion or all of these failed to produce berries. In the ensuing year both flowers of each pair were invariably fertilised in the same manner; and the latter experiments alone serve to show the proportion of flowers which yield berries, when legitimately and illegitimately fertilised; but for calculating the average number of seeds per berry I have used those produced during both seasons.
In the long-styled flowers the stigma projects just above the bearded throat of the corolla, and the anthers are seated some way down the tube. In the short- styled flowers those organs occupy reversed positions. In this latter form the fresh pollen-grains are a little larger and more opaque than those of the long- styled form. The results of my experiments are given in Table 3.21.
TABLE 3.21. Mitchella repens.
Column 1: Nature of the Union. Column 2: Number of Pairs of Flowers fertilised during the second season. Column 3: Number of Drupes produced during the second season. Column 4: Average Number of good Seeds per Drupe in all the Drupes during the two Seasons.
Long-styled by pollen of short-styled. Legitimate union : 9 : 8 : 4.6.
Long-styled by own-form pollen. Illegitimate union : 8 : 3 : 2.2.
Short-styled by pollen of long-styled. Legitimate union: 8 : 7 : 4.1.
Short-styled by own-form pollen. Illegitimate union : 9 : 0 : 2.0.
The two legitimate unions together : 17 : 15 : 4.4.
The two illegitimate unions together : 17 : 3 : 2.1.
It follows from this table that 88 per cent of the paired flowers of both forms, when legitimately fertilised, yielded double berries, nineteen of which contained on an average 4.4 seeds, with a maximum in one of 8 seeds. Of the illegitimately fertilised paired flowers only 18 per cent yielded berries, six of which contained on an average only 2.1 seeds, with a maximum in one of 4 seeds. Thus the two legitimate unions are more fertile than the two illegitimate, according to the proportion of flowers which yielded berries, in the ratio of 100 to 20; and according to the average number of contained seeds as 100 to 47.
Three long-styled and three short-styled plants were protected under separate nets, and they produced altogether only 8 berries, containing on an average only 1.5 seed. Some additional berries were produced which contained no seeds. The plants thus treated were therefore excessively sterile, and their slight degree of fertility may be attributed in part to the action of the many individuals of Thrips which haunted the flowers. Mr. J. Scott informs me that a single plant (probably a long-styled one), growing in the Botanic Gardens at Edinburgh, which no doubt was freely visited by insects, produced plenty of berries, but how many of them contained seeds was not observed.
Borreria, nov. sp. near valerianoides (Rubiaceae).
Fritz Muller sent me seeds of this plant, which is extremely abundant in St. Catharina, in South Brazil; and ten plants were raised, consisting of five long- styled and five short-styled. The pistil of the long-styled flowers projects just beyond the mouth of the corolla, and is thrice as long as that of the short-styled, and the divergent stigmas are likewise rather larger. The anthers in the long-styled form stand low down within the corolla, and are quite hidden. In the short-styled flowers the anthers project just above the mouth of the corolla, and the stigma stands low down within the tube. Considering the great difference in the length of the pistils in the two forms, it is remarkable that the pollen-grains differ very little in size, and Fritz Muller was struck with the same fact. In a dry state the grains from the short-styled flowers could just be perceived to be larger than those from the long-styled, and when both were swollen by immersion in water, the former were to the latter in diameter in the ratio of 100 to 92. In the long-styled flowers beaded hairs almost fill up the mouth of the corolla and project above it; they therefore stand above the anthers and beneath the stigma. In the short-styled flowers a similar brush of hairs is situated low down within the tubular corolla, above the stigma and beneath the anthers. The presence of these beaded hairs in both forms, though occupying such different positions, shows that they are probably of considerable functional importance. They would serve to guard the stigma of each form from its own pollen; but in accordance with Professor Kerner's view their chief use probably is to prevent the copious nectar being stolen by small crawling insects, which could not render any service to the species by carrying pollen from one form to the other. (3/23. 'Die Schutzmittel der Bluthen gegen unberufene Gaste' 1876 page 37.)
The flowers are so small and so crowded together that I was not willing to expend time in fertilising them separately; but I dragged repeatedly heads of short-styled flowers over three long-styled flower-heads, which were thus legitimately fertilised; and they produced many dozen fruits, each containing two good seeds. I fertilised in the same manner three heads on the same long- styled plant with pollen from another long-styled plant, so that these were fertilised illegitimately, and they did not yield a single seed. Nor did this plant, which was of course protected by a net, bear spontaneously any seeds. Nevertheless another long-styled plant, which was carefully protected, produced spontaneously a very few seeds; so that the long-styled form is not always quite sterile with its own pollen.
Faramea [sp.?] (Rubiaceae).
Fritz Muller has fully described the two forms of this remarkable plant, an inhabitant of South Brazil. (3/24. 'Botanische Zeitung' September 10, 1869 page 606.) In the long-styled form the pistil projects above the corolla, and is almost exactly twice as long as that of the short-styled, which is included within the tube. The former is divided into two rather short and broad stigmas, whilst the short-styled pistil is divided into two long, thin, sometimes much curled stigmas. The stamens of each form correspond in height or length with the pistils of the other form. The anthers of the short-styled form are a little larger than those of the long-styled; and their pollen-grains are to those of the other form as 100 to 67 in diameter. But the pollen-grains of the two forms differ in a much more remarkable manner, of which no other instance is known; those from the short-styled flowers being covered with sharp points; the smaller ones from the long-styled being quite smooth. Fritz Muller remarks that this difference between the pollen-grains of the two forms is evidently of service to the plant; for the grains from the projecting stamens of the short-styled form, if smooth, would have been liable to be blown away by the wind, and would thus have been lost; but the little points on their surfaces cause them to cohere, and at the same time favour their adhesion to the hairy bodies of insects, which merely brush against the anthers of these stamens whilst visiting the flowers. On the other hand, the smooth grains of the long-styled flowers are safely included within the tube of the corolla, so that they cannot be blown away, but are almost sure to adhere to the proboscis of an entering insect, which is necessarily pressed close against the enclosed anthers.
It may be remembered that in the long-styled form of Linum perenne each separate stigma rotates on its own axis, when the flower is mature, so as to turn its papillose surface outwards. There can be no doubt that this movement, which is confined to the long-styled form, is effected in order that the proper surface of the stigma should receive pollen brought by insects from the other form. Now with Faramea, as Fritz Muller shows, it is the stamens which rotate on their axes in one of the two forms, namely, the short-styled, in order that their pollen should be brushed off by insects and transported to the stigmas of the other form. In the long-styled flowers the anthers of the short enclosed stamens do not rotate on their axes, but dehisce on their inner sides, as is the common rule with the Rubiaceae; and this is the best position for the adherence of the pollen-grains to the proboscis of an entering insect. Fritz Muller therefore infers that as the plant became heterostyled, and as the stamens of the short- styled form increased in length, they gradually acquired the highly beneficial power of rotating on their own axes. But he has further shown,