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Show 2 INTRODU01'0RY REMARKS. UIIAT'. I. in which the pollen is mature before the stigma, and proterogynous species, in which the r~verso o •tus ; this latter forn1 of dichogan1y not be1ng. nearly so common as the other. Cross-fertilisation 1~ also en-. sured, in many c~ses, by mechanical contn.vanc s of wonderful beauty, preventing the impregnat1on of th . flowers by their own pollen. . There. is a s1na~l clas · _of plants, which I have called d1mo:·ph1c and trnnorph1c, but to which Hildebrand. has g1ven tho more a1 propriate name of heterostyled; t_hi~ class c nsists of plants presenting two or three d1st1nc~ forms, adap~od for reciprocal fertilisation, so that, hke plants w1th separate sexes, they can hardly fail to be int rcrossed in each generation. The male and female organs of some flowers are irritable, and the insects whi h touch thmn get dusted with pollen, which is thus tran ported to other flowers. Again, there is a class, in which tho ovules absolutely refuse to be fertilised by poll n fr~m the sarne plant, but can be fertilised by poll n from any other individual of the san1e species. Th re an' also very 1nany species which are partially sterile with their own pollen. Lastly, there is a large class in whieh the flowers present no apparent obstacle of any kind to self-fertilisation, nevertheless these plants are frequently intercrossed, owing to the prepotency of pollen from another individual or variety over tho plant's own pollen. . A.s plants are adapted by such diversified an l ffecti ve means for cross-fertilisation, it might have be "n inferred frorn this fact alone that they derived so1n great advantage from the process; and it is tho object of the present work to show the nature and i1nportancc of the benefits thus derived. There are, howoyor, some exceptions to the rule of plants being constructecl so as to allow of or to favour cross-fertilisation for son1o ' 01-IAP, l. INTRODUCTORY REMARKS. 3 fe,v plants seem to be invariably self-fertilised; yet even these retain traces of having been formerly adapted for cross-fertilisation. These exceptions need not make u~ doubt the truth of the above rule, any n1ore than the existence of some few plants which produce flowers, and yet never set seed, should 1nake us doubt that flowers are adapted for the production of seed and the propagation of the species. We should always keep in 1nind the obvious fact that the production of seed is the chief end of the ' act of fertilisation ; and that this end can be gained by hermaphrodite plants with incomparably greater certainty by self-fertilisation, than by the union of the sexual elements belonging to two distinct flowers or plants. Yet it is as unmistakably plain that innumerable flowers are adapted for cross-fertilisation, as that the teeth and talons of a carnivorous animal are ·adapted for catching prey; or that the plumes, wings, and hooks of a seed are adapted for its dissemination. Flowers, therefore, are constructed so as to gain two objects which are, to a certain extent, antagonistic, and this explains many apparent anomalies in their structure. The close proximity of the anthers to the stigma in a multitude of species favours, and often leads, to self-fertilisation; but this end could have been gained far 1nore safely if the flowers had been completely closed, for then the pollen would not have been injured by the rain or <levoured by insects, as often happens . Moreover, in this case, a very small ·quantity of pollen would have been sufficient for fertilisation, instead of 1nillions of grains being produced. But the openness of the flower and the production of a great and apparently wasteful amount of pollen are necessary for cross-fertilisation. These remarks are well illustrated by the plants called cleistogene, which bear on the B 2 |
