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Show 186 HETEROSTYLED TRIMORPHIC PLANTS. CHAP. IV. mid -sty led forms. The three anthers of each se~ of stamens, more especially those of the shortest stamens, are placed one beneath the other, and the ends of the filaments are bowed a little upwards, so that the pollen from all the anthers would be effectively brushed off by the proboscis of a visi~ing insect. . The relative diameters of the pollen-grains, after having been long soaked in water, are given in the following list, as measured by my son Francis. Divi!•ions of the .Micrometer. Long-styled form, from the mid-length stamens . • . 13•2 " " (Average of 20 measurements.) from the shortest stamens . . (10 measurements.) Mid-styled form, from the longest stamens • (l5 measurements.) " " from the shortest stamens . (20 measurements.) Short-styled form, from the longest stamens . " " (20 measurements.) from the mid-length stamens . (20 measurements.) 9•0 • • 16 •4 r, we have here the usual rule of the grains frmn the longer stamens, the tubes of which have to penetrate the longer pistil, being larger than those fron1 the stamens of less length. 'rhe extreme difference In diameter between the grains from the longest stainens of the mid -sty led form, and from the shortest stain ens of the long-styled, is as 16 · 4 to 9 · 0, or as 100 to 5~; and this is the greatest difference observed by me In any heterostyled plant. It is a singular fact that the grains from the corresponding longest stainens in the two forms differ considerably in diameter; as do those in a lesser degree from the corresponding mid-length stamens in the two forms · whilst those from the corresponding shortest stam:ns in the long- and. In~dstyled forms are almost exactly ·equal. TheH ~nequality in the two first cases depends on the grains CHAP. IV. 'P ON'rEDERIA. 187 in both sets of anthers in the short-styled form being smaller than those from the corresponding anthers in the other two forms ; and here we have a case parallel with that of the mid-styled form of Lythrum salicaria. In this latter plant the pollen-grains of the mid-styled forms are of smaller size and have less fertilising power than the corresponding ones in the other two forms ; whilst the ovarium, however fertilised, yields a greater number of seeds; so that the mid-styled form is altogether more feminine in nature than the other two forms. In the case of Pontederia, the ovarium includes only a single ovule, and what the meaning of the difference in size between the pollen -grains from the corresponding sets of anthers may be, I will not pretend to conjecture. The clear evidence that the species just described is heterostyled and trimorphic is the more valuable as there is some doubt with respect to P. cordata, an inhabitant of the United States. Mr. Leggett suspects* that it is either diinorphic or trimorphic, for the pollen-grains of the longer stamens are "more than twice the diameter or than eight times the mass of the grains of the shorter stamens. Though minute, these smaller grains seem as perfect as the larger ones." On the other hand, he says that in all the mature flowers, "the style was as long at least as the longer stamens;" "whilst in the young flowers it was intermediate in length between the two sets of stamens;" and if this be so, the species can hardly be heterosty led. * '·Bull. of the Torrey Botanical Club,' 1875, vol. vi. p. 62. |