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Show 342 SELF-STERILE PLANTS. CHAP. JX. 1. n th e genus Passi'flor·a , and with the V andeoo a1nongst Orchids. · d' .[!{' S lf- t ··l't differs much in degree In 1uercnt e s en 1 v . h' h ll p1 a nt s. 1n those extraordinary cas.e s In w IC po en 1 .1 . f. th e ft ro1n e sam 0 , ,..,. , er acts on the stigma 1 re a po. 1son, 't · 1 t certain that the plants would never y1eld a l IS a mos 1. k Q d z· single self-fertilised seed. Other plants, 1 e ory a 1,s cava, OccasionallvJ' though very rarely, prfo duce . a few self-fertilised seeds. A large numb~r o. spec~es, as n1ay be seen in Table F, are less fertile with then own pollen than with that from anothe_r plant ; and. lastly, 801ne species are perfectly self-~ertile. .Even w1th the individuals of the sa1ne Rpecies, as JUSt re1narked, some are utterly self-sterile, others moderately ~o, and some perfectly self-fertile. The cause, whatever 1t m~y be which renders 1nany plants 1nore or less stenle wi~h their O"\Vll pollen, that is, when they . arc selffertilised 1nust be different, at least to a certain extent, from tha~ ,which detennines the difference in height, \igour, and fertility of the seedlings raised fr01n selffertilised and cross cl s eds; for we have already seen that the two classes of cases do not Ly any n1eaus run l)arallel. This want of parallelism w?uld .. be intelligible, if it coulcl be shown that self-stenhty depended solely on the incapacity of the pollen-tubes to penetrate the stigma of the san1e flower deeply enough to reach the ovules; whilst the greater or less yigorous growth of the seecllings no doul>t clep~nds on the nature of the contents of the pollen-grains and ovules. Now it is certain that with son1e plants the stig1natic secretion does not properly excite the pollengrains, so that the tubes are not properly Jevelope~, if the pollen is taken from the same flower. This .1s the case according to ~-,ritz 1\iiiller with Eschscholtzw, for he found that the po lle n-tub es dI.d n ot penetrate CHAI). IX. SELF-STERILE PLANTS. 343 the stigma deeply ;* and with the Orchidaceous O'enns N oty lia they failed altogether to penetrate it. b With di1norphic and trin1orphic species, an illegitimate union between 17lants of the same fonn presents the closest analogy \vith self-fertilisation, whilst a legitimate union closely resembles cross-fertilisation· and. _here aga~n the lessened fertility or co1nplet~ stenhty of an Illegitimate union depends, at least in part, on the incapacity for interaction between the pollen-grains and stigma. Thus with Linum grandi-florun~, as I have elsewhere shown,t not more than two or three out of hundreds of pollen-grains, either of the long-sty l eel or short-sty led torm, when placed on the stig1na of their own form, e1nit their tubes, and these do not penetrate deeply; nor does the stigma itself change. colour, as occurs when it is legitimately fertilised. On th8 other hand the difference in innate fertility as well as in growth between plants raised fron1 cross~cl and self-fertilised seeds, and the difference in fertility and ~rowth between the legitimate and illegitimate offspnng 0£' di1norphic and trin1orphic plants, must depend on some inco1npatibility between the sexual ele~ne~ts contained within the pollen-grains and ovules, as 1t IS through their union that new organisms are developed. If we now turn to the more im1nediate cause of self-sterility, we clearly see that in n1ost ca8es it is detennine~ by the conditions to which the plants have bee~ su?Jected. ~rhus Eschscholtzia is completely selfster~ le In the hot clin1ate of Brazil, but is perfectly fertile there with the pollen of any other individual. The offspring of Brazilian plants becan1e in England ll~.' Bot. Zcitung,' 18G8, PI; 114-, --t-;Journal of Linn. l'oc. Bot.' vol. vii. 1Sti3, pp. 73-75. |