OCR Text |
Show CONCLUDING REMARKS CHAP. VI. spread from a single centre to such widely remote and separated areas. The family of the Rubiacem contains not far short of as many heterostyled genera as all the other thirteen families together ; and hereafter no doubt other Rubiaceous genera will be found to be heterostyled, although a large majority are homostyled. Several closely allied genera in this family probably owe their heterostyled structure to descent in common; but as the genera thus characterised are distributed in no less than eight of the tribes into which this family has been eli vided by Bentham and Hooker, it is almost certain that several of them must have become heterostyled. independently of one another. What there is in the constitution or structure of the me1nbers of this fa1nily which favours their beco1ning heterostyled, I cannot conjecture. Some families of considerable size, such as the Boraginere and Verbenacere, include, as far as is at present known, only a single heterostyled genus. Polygonum also is the sole heterostyled genus in its family; and though it is a very large genus, no other species except P. jagopyrum is thus characterised. vVe may suspect that it has become heterostyled within a comparatively recent period, as it seems to be less strongly so in function than the species in any other genus, for both forms are capable of yielding a considerable number of spontaneously self-fertilised seeds. Polygonum in possessing only a single heterostyled species is an extreme case; but every other genus of considerable size which includes some such species likewise contains homostyled species. Lythrum includes trimorphic, dimorphic, and ho1nostyle<i species. Trees, bushes, and herbaceous plants, both large and small, bearing single flowers or flowers in dense spikes or heads, have been rendered heterostyled. . CHAP. VI. ON HETEROSTYLED PLANTS . 257 So have plants which inhabit alpine and lowland sites, dry land, marshes and water.* When I first began to experimentise on hetergstyled plants it was under the impression that they were tending to become dioocious; but I was soon forced to relinquish this notion, as the long-styled plants of Primula which, from possessing a longer pistil, larger ·stigma, shorter sta1nens with smaller pollen-grains, seemed to be the more feminine of the two forms, yielded fewer seeds than the short-styled plants which appeared to be in the above respects the more masculine of the two. Moreover, trimorphic plants evidently come under the same category with dimorphic, and tl~e former cannot be looked at as tending to become direcious. With Lythru1n salicaria, however, we have the curious and unique case of the mid-styled fonn being more feminine or less masculine in nature than the other two forms. This is shown by the large * Out of the 38 genera known to include heterostyled species, about eight, or 21 per cent., are mor.e or less aquatic in their habits. I was at first struck with this fact, for I was not then a ware h?w large a proportion of ordmary plants inhabit such stations.. ~Ieterostyled plants may be sa1d m one sense to have their sexes separated, as the forms must mutually fertilise one another. Therefore it seemed worth while to ascertain what proportion of the genera in the Linnean classes Mon_oocia, Dioocia and Poly~ ~am1a, contained" species which hve "in water, marshes, bogs or watery places." In Sir W. J. H?<>ker's 'British Flora' (4th ed1t. 1838) these three Linnean classes include 40 genera, 17 of which (i.e. 43 per cent.) contain species inhabiting the just-specified stations. So that 43 per cent. of those British plants which have their sexes separated are more or less aquatic in their habits, whereas only 21 per cent. of heterostylcd plants have such habits. I may add that the hermaphrodite classes, from Monandria to Gynandria inclusive, contain 447 genera, of which 113 are aquatic in the above sense, or only 25 per cent. It thus appears, as far as can be judged from such imperfect data, that there is some conneption bc:tween the separation of the sexes in plants and the watery nature of the sites which they inhabit ; but that this does not hold gooJ with heterostyled specie:!. |