448 REPRODUCTION. 



internal, under which the reproductive elements have been pro- 

 duced are widely dissimilar. 



The mechanism by which close-fertilization is secured in some 

 instances and absolutely prevented in others has been fully 

 explained in Volume I. The account of the mechanism is now 

 to be supplemented by a statement of the results of reproduction 

 in the different degrees of relationship. 



1151. The results of close-fertilization contrasted with those 

 of cross-fertilization. It has long been known to cultivators of 

 plants, that in order to keep the desirable varieties which are 

 under cultivation " true to seed " they must be close bred ; that 

 is, all pollen from other varieties of the same species must be 

 excluded. The whole subject is best illustrated by reference to 

 the numerous experiments by Darwin ; the exhaustive nature of 

 which is indicated by an account of a single series given nearly 

 in his own words. 



1152. The plants experimented upon in all cases were raised 

 from carefully ripened seed, and, when ready to flower, were 

 placed under nets with meshes of one tenth of an inch in diame- 

 ter, in order that all pollen-carrying insects might be excluded. 



A plant of Ipomoea purpurea (Morning Glory) , growing in the 

 greenhouse, was protected in the manner just described, after ten 

 of its flowers had been fertilized by pollen from their own sta- 

 mens, and ten others by pollen from a distinct plant of the same 

 species. The seeds from the first ten flowers may be termed 

 self-fertilized, those from the other ten, crossed. The two kinds 

 of seeds were placed on damp sand on opposite sides of a glass 

 tumbler covered by a glass plate, with a partition between the 

 seeds, and the glass was put in a warm place. As often as a pair 

 of seeds germinated the} T were put on opposite sides of a pot, 

 with a superficial partition between them, and the same procedure 

 was followed until five or more seedlings of exactly the same age 

 were planted on the opposite sides of several pots. The soil in 

 the pots in which the plants grew was well mixed, and the plants 

 on the two sides were always watered at the same time ; thus the 



f 



seedlings were subjected to practically the same conditions from 

 a very early stage. 



In the same manner self-fertilized and crossed seeds were 

 secured during ten generations. The results, so far as these can 

 be shown by measurement of the plants, are exhibited in the 

 following table : l - 



1 Darwin : Effects of Cross and Self Fertilization, 1876, p. 52. 



