OCR Text |
Show 258 CONCLUDING REMARKS CHAP. VI. number of seeds which it yields in whatever 1nannm· it may be fertilised, and by its pollen (the grains of which are of smaller size than those from the corresponding stamens in the other two forms) when applied to the stigma of any form. producing fewer seeds than the normal nun1bor. If we suppose the process of deterioration of the male organs in the miltstyled form to continue, the final result would be the 1)roduction of a female plant; and Lythru1n salicaria would then consist of two heterostyled hel'maphrodites and a female. No such case is known to exist, but it is a possible one, as hennaphrodite and female forms of the same species are by no 1neans rare. Although there is no reason to believe that hotorostylecl 1)lants are regularly becoming dicecious, yet they offer sing~ lar facilities, as will hereafter be shown, fm· such conversion; and this appears occasionally to have been effected. We may feel sure that plants have been rendered heterostyled to ensure cross-fertilisation, for we now know that a cross between the distinct indivicluals of the same species is highly important for the vigour and fertility of the offspring. The sa1ne end is gained by dichogamy or the maturation of the reproductive elements of the same flower at, different periods,-by dioociousness-self-sterili ty-t~e prepotency of pollen from another individual over a plant's own pollen,-and lastly, by the structure of the flower in relation to the visits of insects. The wonderful diversity of the means for gaining the same end in this case, and in rn.any others, depends on the nature of all the prevwus changes through which the species has passed, ancl.on the more or less complete inheritance of the succ.e~sive adaptations of each part to the surrounding conditwns. CHAP. VI. ON HETEROSTYLED PLANTS. 259 Plants which are already well adapted by the structure of their flowers for cross-fertilisation by the aid of insects often possess an irregular corolla, which has been modelled in relation to their visits; and it would have been of little or no use to such plants to have become heterostyled. We can thus understand why it is that not a single species is heterostyled in such great families as the Leguminosc:e, Labiatm, Scrophulariacem, Orchidem, &c., all of which have irregular flowers. Every known heterostyled plant, however, depends on insects for its fertilisation, and not on the wind ; so that it is a rather surprising fact that only one genus, Pontederia, has a plainly irregular corolla. Why some species are adapted for cross-fertilisation, whilst others within the same genus are not so, or if they once were, have since lost such adaptation and in consequence are now usually self-fertilised, I have endeavoured elsewhere to explain to a certain limited extent.* If it be further asked why some species have been adapted for this end by being made heterostyled, rather than by any of the a.hJve specified means, the answer probably lies in the manner in which heterostylism originated,-a subject immediately to be discussed. Heterostyled species, however, have an advantage over dichogamous species, as all the flowers on the same heterostyled plant belong to the same form, so that when fertilised legitimately by insects two distinct individuals. are sure to intercross. On the other hand, with dichogamous plants, early or late flowers on the same individual may intercross; and a cross of this kind does hardly any or no good. Whenever it is profitable to ~ species to produce a * 'The Effects of Cross and E:elf-fertilisation,' 1876, p. 441. s 2 |