Lythrum includes trimorphic, dimorphic, and homostyled species.
Trees, bushes, and herbaceous plants, both large and small, bearing single flowers or flowers in dense spikes or heads, have been rendered heterostyled. So have plants which inhabit alpine and lowland sites, dry land, marshes and water. (6/3. Out of the 38 genera known to include heterostyled species, about eight, or 21 per cent, are more or less aquatic in their habits. I was at first struck with this fact, for I was not then aware how large a proportion of ordinary plants inhabit such stations. Heterostyled plants may be said in 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, Monoecia, Dioecia and Polygamia, contained species which live "in water, marshes, bogs or watery places." In Sir W.J. Hooker's 'British Flora' 4th edition 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 heterostyled 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 connection between the separation of the sexes in plants and the watery nature of the sites which they inhabit; but that this does not hold good with heterostyled species.)
When I first began to experimentise on heterostyled plants it was under the impression that they were tending to become dioecious; 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 stamens 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 the former cannot be looked at as tending to become dioecious. With Lythrum salicaria, however, we have the curious and unique case of the mid-styled form being more feminine or less masculine in nature than the other two forms. This is shown by the large number of seeds which it yields in whatever manner 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 number. If we suppose the process of deterioration of the male organs in the mid-styled form to continue, the final result would be the production of a female plant; and Lythrum salicaria would then consist of two heterostyled hermaphrodites and a female. No such case is known to exist, but it is a possible one, as hermaphrodite and female forms of the same species are by no means rare. Although there is no reason to believe that heterostyled plants are regularly becoming dioecious, yet they offer singular facilities, as will hereafter be shown, for 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 individuals of the same species is highly important for the vigour and fertility of the offspring. The same end is gained by dichogamy or the maturation of the reproductive elements of the same flower at different periods,--by dioeciousness--self-sterility--the 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 many others, depends on the nature of all the previous changes through which the species has passed, and on the more or less complete inheritance of the successive adaptations of each part to the surrounding conditions.