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Show 3-±-± SELF-STERILE PLANTS. CHAP. IX. in a, single generation partially. self-fertile, and still Inore so 1. n tl1 e s econd generation. Con.v ers ly, the o.uti's pn• ng. onf Enoo· lish plants' after grow1ng . for tw. o seasons 1n raz1'1 , bec'a me in the fir.s t . g. ener.a tion. qtnte. lf t se -s en.1 e . Agai·n , Abutilon darw%n~~. , wh1ch 1s ·elf-t s en·1 e 1· n ·t nat1've ho1ne of Brar.Il, became Inodo- 1 s c • • • rately self-fertile in a single generation l? an ~nghsh h ot ho use. Som- e other plants are self-stenle dunng the 1 t Of the Year and later in the season bec01ne ear y par ' . . . se lf- f ert I. le . Passi:fylv ora alata lost .1 ts self-stenhty when rafted on another species. W 1th Reseda, however, ?m whw' h some indiviCluals of the same parentage aT. e self-sterile and others are self-fertile, we are forced In our ignorance to speak of the cause as due to spontaneous variability; but we should remember that the progenitors of these plants, either on the 1nale or femah~ side, may have been exposed to s?n1ewhat . different conditions. The power of the envnonment thus to affect so readily and in so peculiar a 1nanner the reproductive orga11s, is a fact which has many i1nportant bearings; and I _h~ve theref?ro thought the foregoing details worth g1v1ng. ] or Instance, the sterility of many animals and plants u~der changed conditions of life, such as confinement, ev1dently comes within the same general principle of the sexual system being easily affected by the environment. It has already been pr<?ved, that a cross between pla~ts which have been self-fertilised or intercrossed dur.mg several generations, having been kept all the tune under closely si1nilar conditions, does not benefit the offspring ; and on the other hand, that ~ cross between plants that have been subjected to d1ff~rm~t conditions benefits the offspring to an extraordmary h degree deoTee. V\T e n1ay therefore ooncl ude t at some , of b differenti. ati. on 1. n the sexual system · sary for IS necos CH.\P. IX. SELF-STERILE PLANT ·. 345 the full fertility of the parent-plants and for the full vi_gour of their offspring. It seen1s also probable that w1t~ _tho~e plants which are capable of complete self-· fertihsat1o_n, the male and female elements and organs already differ to an extent sufficient to excite thel.r mutual interaction ; but that when such plants are taken t~ anoth:-r country, and become in consequence self-sterile, theu sexual elements and organs are so acted on as to be rendered too uniform for such interacti~ n, like those of a self-fertilised plant long culttvated u~der the same conditions. Conversely, we 1nay further Infer that plants which are self-sterile in their native country, but become self-fertile under changed conditions, have their sexual ele1nents so acted on, that they become sufficiently differentiated for mutual interaction. We know that self-fertilised seedlings are inferior in tnany respects to those from a cross· and as with plants in a state of nature pollen fron1 the sarne flowei: can hardly ~ail to ?e often left by insects or by the Wind on the .stig~~' It seems at first sight highly probable that self-stenhty has been gradually acquired thr~~gh . natural selection in order to prevent selffertilisation. It is no valid objection to this belief that ~~e structure of some flowers, and the dichogamous condi~Ion of many others, suffice to prevent the pollen reaching the stigma of the same flower; for we should remember that with most species many flowers expand at the saine time, and that pollen from the same plant is equally injurious or nearly so as that from the saine flower. Nevertheless the belief !hat ~:If-sterility is a q.uality which has be:n gradually .cq~~red_ for the spec1al purpose of preventing selffertilisation 1nust, I believe, be rejected. In the first place, there is no close correspondence in degree |
