This section is from the book "On British Wild Flowers Considered In Relation To Insects", by John Lubbock. Also available from Amazon: Nature Series On British Wild Flowers Considered In Relation To Insects.
There are other points in which the two forms differ from one another; for instance, the stigma of the long-styled form is globular and rough, while that of the short-styled is smoother, and somewhat depressed. The pollen of the two forms (Figs. 38 and 39) is also dissimilar; that of the long-styled being considerably smaller than the other - 7/1000 of an inch in diameter against 10-11/7000 or nearly in the proportion of three to two; a difference, the importance of which is probably due to the fact that each grain has to give rise to a tube which penetrates the whole length of the style, from the stigma to the base of the flower; and the tube which penetrates the long-styled pistil must therefore be nearly twice as long as in the other. Mr. Darwin has shown that much more seed is set, if pollen from the one form be placed on the pistil of the other, than if the flower be fertilised by pollen of the same form, even taken from a different plant. Nay, what is most remarkable, such unions in Primula are more sterile than crosses between some nearly allied, though distinct species of plants.
The majority of species of the genus Primula appear to be dimorphous, but this is not the case. (Scott, "Proc. Linn. Soc." vol. viii. 1864, p. 80.)
Mr. Darwin has since pointed out ("Jour. Linn. Soc." 1863) that several species of Linum are dimorphous, in the same manner as those of Primula; and has shown that the existence of three forms in Lythrum (Figs. 77 - 80) already observed by Vaucher, is to be explained in the same manner. I shall refer to this case more in detail when we come to that family. Nor are these by any means the only cases of Heteromorphism now known. I have already mentioned that of Oxalis, and Hildebrand gives the following list of genera as containing Heteromorphous species, viz., Hottonia, Primula, Linum, Lythrum, Pulmonaria, Cinchona, Mitchella, Plantago, Rhamnus, Amsinckia, Mertensia, Leucosmia, Drymospermum,
Menyanthes. and Polygonum. It will be observed that these genera belong to very different groups, while on the other hand, in several cases, as in Primula itself (Scott, "Proc. Linn. Soc." vol. viii.), we find monomorphous and heteromorphous species in the same genus.
Another point of great interest is the spontaneous movement of the stamens and pistil in dichogamous plants, first observed by Kolreuter in Ruta graveo-lens; he, however, supposed that the object was to bring the stamens in contact with the pistil; whereas the real advantage, as Sprengel pointed out, is that in consequence the stamens and pistil successively occupy the same spot in the flower, and thus come in contact with the same part of the insect. For instance, in Geranium pratense (Fig. 40), when the flower first opens, the stamens lie on the petals, at right angles with the upright pistils. As, however, they come to maturity they raise themselves (Fig. 41 a), parallel and close to the pistil (Fig. 41 b), which, however, is not as yet capable of fertilisation. After they have shed their pollen, they return to their original position (Fig. 42), and the stigmas unfurl themselves. More or less similar movements have been observed in various other flowers. Thus the cells of the anthers of the Foxglove (Digitalis purpurea) (Figs 100 - 102), which are at first transverse, become longitudinal as they ripen.
Fig. 40. - Geranium pratense.
Fig. 41. - Geranium pratense (young flower). Five of the stamens are erect.
Fig. 42. - Geranium pratense (older flower). The stamens have retired, and the stigmas are expanded.
In aquatic plants, the blossoms habitually come to the surface. In Valisneria spiralis the female flower has a long spiral stalk which enables it to rise to the top of the water. The male flowers which are small, very numerous, and attached lower down, separate themselves altogether from the plant, rise to the surface, and fertilise the female flowers, among which they float. When this is effected, the spiral stalk of the female flower again contracts, and draws it down below the surface.
While the pollen grains from each flower agree very closely with one another, those of different species differ greatly in form, size, character of surface, etc. Doubtless there are reasons for these differences, but the subject is one with reference to which we have as yet very little information.
According to Sprengel, the pollen of wind-fertilised flowers is drier, and therefore more easily carried by the wind, than that of most insect-fertilised flowers. I say of most, because in some cases, for instance in the violet, as will be shown presently, it is as necessary that the pollen should separate readily from the anthers, as in wind-fertilised flowers.
Mr. Bennett states that the pollen of wind-fertilised flowers is generally spherical; while that of insect-fertilised flowers is usually furrowed, the furrows running along the longer axis of the grain.
In Dimorphous species the pollen of the short-styled form is generally larger than that of the long-styled form, but in Linum, according to Hildebrand, ("Die Ges. Verth. bei den Pflanzen," p. 37) it is of the same size in both forms.
In Faramea, another Dimorphous group, the surface of the pollen grains is different in the two forms (Thome "Das Gesetz der vermiedenen Selbstbe-fruchtung bei den hoheren Pflanzen," 1870), the smaller grains of the long-styled form are studded with small points; in consequence of which the pollen-grains are less easily detached from the anther; this difference possibly has reference to the different position of the two forms; the smooth ones being sheltered by the flower; while the larger pollen-grains, which are produced in the anthers of the long stamens, and are therefore more exposed to the wind, are, in consequence of their roughness, less liable to be blown away and wasted.