418 The Biology of Flowers 



cross-fertilisation now appear to be useful adaptations. Darwin was, 

 however, far from giving undue prominence to this point of view, 

 though this has been to some extent done by others. He particularly 

 emphasised the following consideration : — " But we should always 

 keep in mind that two somewhat opposed ends have to be gained ; 

 the first and more important one being the production of seeds by 

 any means, and the second, cross-fertilisation 1 ." Just as in some 

 orchids and cleistogamic flowers self-pollination regularly occurs, 

 so it may also occur in other cases. Darwin showed that Pisum 

 sativum and Lathyrus odoratus belong to plants in which self- 

 pollination is regularly effected, and that this accounts for the 

 constancy of certain sorts of these plants, while a variety of form 

 is produced by crossing. Indeed among his culture plants were 

 some which derived no benefit from crossing. Thus in the sixth 

 self-fertilised generation of his Ipomoea cultures the "Hero" made 

 its appearance, a form slightly exceeding its crossed companion in 

 height ; this was in the highest degree self-fertile and handed on its 

 characteristics to both children and grandchildren. Similar forms 

 were found in Mimulus luteus and Nicotiana 2 , types which, after 

 self-fertilisation, have an enhanced power of seed-production and of 

 attaining a greater height than the plants of the corresponding 

 generation which are crossed together and self-fertilised and grown 

 under the same conditions. "Some observations made on other 

 plants lead me to suspect that self-fertilisation is in some respects 

 beneficial ; although the benefit thus derived is as a rule very small 

 compared with that from a cross with a distinct plant 3 ." We are as 

 ignorant of the reason why plants behave differently when crossed 

 and self-fertilised as we are in regard to the nature of the differentia- 

 tion of the sexual cells, which determines whether a union of the 

 sexual cells will prove favourable or unfavourable. 



It is impossible to discuss the different results of cross-fertilisa- 

 tion; one point must, however, be emphasised, because Darwin 

 attached considerable importance to it. It is inevitable that pollen 

 of different kinds must reach the stigma. It was known that pollen 

 of the same " species " is dominant over the pollen of another species, 

 that, in other words, it is prepotent. Even if the pollen of the same 



1 Cross and Self fertilisation (1st edit.), p. 371. 



2 In Pisum sativum also the crossing of two individuals of the same variety produced 

 no advantage ; Darwin attributed this to the fact that the plants had for several generations 

 been self- fertilised and in each generation cultivated under almost the same conditions. 

 Tschermak ("TJeber kiinstliche Kreuzung an Pisum sativum") afterwards recorded the 

 same result; but he found on crossing different varieties that usually there was no 

 superiority as regards height over the products of self- fertilisation, while Darwin found 

 a greater height represented by the ratios 100 : 75 and 100 : 60. 



3 Cross and Self fertilisation, p. 350. 



