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Show -166 GENERAL RESULTS. CHAP. XII. a species fertilised with pollen from a distinct genu~. rrhe deoTee of sterility is much affected by the COnditions to 5which the plants have been subjected.* rrhc pollen from a distinct form is strongly prepotent over that from the sa1ne form, although the former may have been placed 6n the stigma many hours afterwards. rrh ffsprino· from a union between plants of the same e 0 form are mob re or less sten. le, h. ke hy b n· a s,. ~n d h ave their pollen in a more or less aborted cond1t1on ; and , f the seedlings are as barren and as dwarfed as ~ome 0 bl h b · d · the 1nost barren hybrid. They also resem e y n. s 1n several other respects, which need not here be speC1~ed in detail,-such as their sterility not corresponding in degree with that of the parent plants,-the u~equal sterility of the latter, when recipr?cally .united,and the varying sterility of the seedhngs ra1secl from the same seed-capsule. . . We thus have two grand classes of cases giving resu.lts which correspond in the most striking manner with those which follow from the crossing of so-cal~ed true and distinct species. vVith respect to the diffe~~nce between s edlin crs raised ·from cross and self fertihsed flowers, there is ~ood evidence that this depends altogether on whether the se~ual ele.ments of t~e parents have been sufficiently differentiated, by ~xp_osure to different condition or by spontaneous vanation. It is probable that nearly the same conclusion may be extended to heterostyled plants; but this is not the proper place for di·s cuss·i ng t h e on·g i·n of the. long-styled ~hort-styled and mid-styled forms, whiCh all belong' to the same species as certai· n1 Y as d 0 the two ' . h sexes of the same species. vVe have therefore no ng t . . · h first to maintain that the stenhty of spec1es w en * ' Journal Linn. Soc. Bot. ' vo 1. .. · 1864 p ISO Vlll. ' • • CHAP. XII. GENERAL RESULTS. - 467 crossed and of their hybrid offspring, is determined b some c~use fundamentally different from that which det~rmines the sterility of the individuals both of ord~nary and of heterostyled plants when united in vanous ways. Nevertheless, I am aware that it will take mai~Y years to remove this prejudice. There 1s hardly anything more wonderful in nature ~han the sensitiveness of the sexual elements to external Influences, and the delicacy of their affinities. We see this in slight changes in the conditions of life beino· fav?urable .to the fertility and vigour of the parent~ while ce:ta1n o~her ~nd not groat changes cause them to be quite stenle Without any apparent injury to their health. We see how sensitive the sexual elements of tho~e plants must be, which are completely sterile with ~he~r. own pollen, but are fertile with that of any other Ir:dividual of the same species. Such plants become either more or less self-sterile if subjected to changed conditions, although the change may be far from great. The ovules of a heterostyled trimorphic plant ar affected very differently by pollen from the three sets of stamens belonging to the same species. With ordinary plants the pollen of another variety or merely of another individual of the same variety is often strongly prepotent over its own pollen, when both are placed at the same time on the same stigma. In those great families of plants containing many thousand allied species, the stigma of each distinguishes with unerring certainty its own pollen from that of every other species. There can be no doubt that the sterility of distinct species when :first crossed, and of their hybrid offspring, depends exclusively on the nature or affinities of their sexual elements. We see thi~ in the want of any close correspondence between the degree 2 H 2 |