414 



The Biology of Flowers 



continued to the tenth generation and the flowers were crossed in 

 different ways. We see, therefore, that this book also represents an 

 enormous amount of most careful and patient original work. 



The general result obtained is that plants produced as the result 

 of cross-fertilisation are superior, in the majority of cases, to those 

 produced as the result of self-fertilisation, in height, resistance to 

 external injurious influences, and in seed-production. 



Ipomoea purpurea may be quoted as an example. If we express 

 the result of cross-fertilisation by 100, we obtain the following 

 numbers for the self-fertilised plants. 



Taking the average, the ratio as regards growth is 100 : 77. The 

 considerable superiority of the crossed plants is apparent in the first 

 generation and is not increased in the following generations ; but 

 there is some fluctuation about the average ratio. The numbers 

 representing the fertility of crossed and self-fertilised plants are 

 more difficult to compare with accuracy ; the superiority of the 

 crossed plants is chiefly explained by the fact that they produce 

 a much larger number of capsules, not because there are on the 

 average more seeds in each capsule. The ratio of the capsules was, 

 e.g. in the third generation, 100 : 38, that of the seeds in the capsules 

 100 : 94. It is also especially noteworthy that in the self-fertilised 

 plants the anthers were smaller and contained a smaller amount of 

 pollen, and in the eighth generation the reduced fertility showed 

 itself in a form which is often found in hybrids, that is the first 

 flowers were sterile 1 . 



The superiority of crossed individuals is not exhibited in the 

 same way in all plants. For example in Eschscholzia californica 

 the crossed seedlings do not exceed the self-fertilised in height and 



1 Complete sterility was not found in any of the plants investigated by Darwin. Others 

 appear to be more sensitive ; Cluer found Zea Mais " almost sterile " after three generations 

 of self-fertilisation. (Cf. Fruwirth, Die Zilchtung der LandwirtschaJ'tlichen Kulturpflanzen, 

 Berlin, 1904, n. p. 6.) 



