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Show 160 HETEROSTYLED TRIMORPHIC PLANTS. CHAP. IV. the twelve longest, twelve mid-length, and twelve shortest stamens acts very differently on each of the three stigmas; so that there are three sets of female and of male organs. Moreover, in most cases the six stamens of each set differ somewhat in their fertilising power from the six corresponding ones in one of the other forms. We may further draw the remarkable conclusion that the greater the inequality in length between the pistil and the set of stamens, the pollen of which is employed for its fertilisation, by so much is the sterility of the union increased. There are no exceptions to this rule. To understand what follows the reader should. look to Tables 23, 24, and 25, and to the diagram Fig. 10, p. 139. In the long-styled form the shortest stamens obviously differ in length from the pistil to a greater degree than do the mid-length stamens ; and the capsules produced by the use of pollen from the shortest stamens contain fewer seeds than those produced by the pollen from the midlength stamens. The same result follows with the long-sty led form, from the use of the pollen of the shortest stamens of the mid -sty led form and of the mid-length stamens of the short-styled form. The same rule also holds good with the mid-styled and short-styled forms, when illegitimately fertilised with pollen from the stamens more or less unequal in length to their pistils. Certain! y the difference in sterility in these several cases is slight; but, as far as we are enabled to judge, it always increases with the increasing inequality of length between the pistil and the stamens which are used in each case. The correspondence in length between the pistil in each form and a set of stamens in the other two forms, is probably the direct result of adaptation, as it is of high service to the species by leading to full and CHAP. IV. LYTHRUM SALICARIA. 161 legitimate fertilisation. But the rule of the increased sterility of the illegitimate unions according to the greater inequality in length between the pistils and stamens employed for the union can be of no service. With some heterostyled dimorphic plants the difference of fertility between the two illegitimate unions appears at first sight to be related to the facility of self-fertilisation; so that when from the position of the parts the liability in one form to self-fertilisation is greater than in the other, a union of this kind has been checked by having been rendered the more sterile of the two. But this explanation does not apply to Lythrum; thus the stigma of the longstyled form is more liable to be illegitimately fertilised with pollen from its own mid-length stamens, or with pollen from the mid-length stamens of the short-styled form, than by its own shortest stamens or those of the mid-sty led form ; yet the two former unions, which it might have been expected would have been guarded against by increased sterility, are much less sterile than the other two unions which are much less likely to be effected. The same relation holds good even in a more striking manner with the mid-styled form, and with the shortstyled form as far as the extreme sterility of all its illegitimate unions allows of any comparison. We are led, therefore, to conclude that the rule of increased sterility in accordance with increased inequality in length between the pistils and stamens, is a purposeless result, incidental on those changes through which the species has passed in acquiring certain characters fitted to ensure the legitimate fertilisation of the three forms. Another conclusion which may be drawn from Tables 23, 24, and 25, even from a glance at them., M |