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. 
Height. Number of seeds. 
1 Generation | 100:76 100 : 64 
2 oi 100 :79 — 
3) es | 100:68 | 100:94 
4 ue 100 : 86 100 : 94 
5 ; 100:75 | 100:89 
6 i 100 :72 — 
7 % 100 : 81 — 
Boog 100 : 85 = 
9 a 100 : 79 100 :26 (Number of capsules) 
10 - 100 :54 = 
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’. 
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 
» 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 Ziuchtung der Landwirtschaftlichen Kulturpflanzen, 
Berlin, 1904, 1. p. 6.) 
