Darwin on the Effects of Cross- and Self- Fertilization. 131 



not those in which the pollen does not reach the stigma un- 

 aided, but those in which it is impotent, or nearly so, when 

 applied, although efficient upon the stigma of another indi- 

 vidual. Verbascum, Pass) flora, Corydalis, and many Orchids 

 afford instances of this sort. In these the advantage of cross- 

 ii rises to a n> ee— ity. A noteworthy fact respecting 

 them (of which Mr. Darwin 'makes much) is, that such self- 

 sterility, or the reverse, is influenced by slight changes in the 

 conditions, such as difference in temperature, grafting on an- 

 other stock, and the like. In S.m P> i ! I ' Muller found 

 that for six <_< m rat mi- al 1 In- plain • \ /.-'/ -/« >'■ " <•,/,/,,,„„ a 

 were completely sterile, unless supplied with pollen from a 

 distinct plain, when theA w ere completely fertile. This was 

 not the case in English' plants, which, when covered by a net, 

 set a considerable number of capsules, the seeds of which, by 

 weight, were as 71 to 100 of those on plants intercrossed by 

 bees. These Brazilian seeds, sent to England, yielded plants 

 with moderately self-fertile flowers, and this limited sell 

 was increased in two generations of English growth. Con- 

 versely, seeds from English plants grown in Brazil were more 

 self-fertile than those reared in Brazil for several generations; 

 yet "one which did not flower the first year, and was thus 

 exposed for two seasons to the climate of Brazil, prm 

 self-sterile, like a Brazilian plant, showing how quic 

 climate had acted on its sexual constitution." Having ob- 

 served that certain individuals of Mignonette were self-sterile, 

 Mr. Darwin secured several such plants under separate nets, 

 and bv inter-crossing these for a few generations, obtained 

 plants whicl • that "without doubt 



tte could easily have been estab- 



Nin'e of the twelve chapters are devoted strictly to the effects 

 of cross and self-fertilization. The tenth considers the "means 

 of fertilization." Cross-fertilization is favored or ensured by: 

 1, the separation of the sexes : : " ma . ie and 



female sexual elements at differ 



even trimorphism: 4. various mechanical contrivances ; 5, the 

 more or less complete inefficiency of a flower's own pollen on 

 its stigma, and the prepotency of pollen from any otht r 

 ual over that from the same plant. We understand that Mr. 

 Darwin is just now occupied in revising and extending his 

 various papers upon these topics, with a view to their publica- 

 tion in a volume. Here he give- a li.t of maets which, when 

 insects are excluded, are either quite sterile or produce less 

 the number of seeds p °ted P Iants - 



This is foil, nits which, when protected from 



insects, are re tlian hal * the num " 



ber of seeds produced by unprotected plants. 



