Autogamy and Geitonogamy 415 



vigour, but the crossing considerably increases the plant's capacity 

 for flower-production, and the seedlings from such a mother-plant 

 are more fertile. 



The conception implied by the term crossing requires a closer 

 analysis. As in the majority of plants, a large number of flowers are 

 in bloom at the same time on one and the same plant, it follows that 

 insects visiting the flowers often carry pollen from one flower to 

 another of the same stock. Has this method, which is spoken of as 

 Geitonogamy, the same influence as crossing with pollen from another 

 plant? The results of Darwin's experiments with different plants 

 (Ipomoea purpurea, Digitalis purpurea, Mimulus luteus, Pelar- 

 gonium, Origanum) were not in complete agreement; but on the 

 whole they pointed to the conclusion that Geitonogamy shows no 

 superiority over self-fertilisation (Autogamy) 1 . Darwin, however, 

 considered it possible that this may sometimes be the case. "The 

 sexual elements in the flowers on the same plant can rarely have 

 been differentiated, though this is possible, as flower-buds are in one 

 sense distinct individuals, sometimes varying and differing from one 

 another in structure or constitution 2 ." 



As regards the importance of this question from the point of view 

 of the significance of cross-fertilisation in general, it may be noted 

 that later observers have definitely discovered a difference between 

 the results of autogamy and geitonogamy. Gilley and Fruwirth 

 found that in Brassica Napus, the length and weight of the fruits as 

 also the total weight of the seeds in a single fruit were less in the 

 case of autogamy than in geitonogamy. With Sinajris alba a better 

 crop of seeds was obtained after geitonogamy, and in the Sugar Beet 

 the average weight of a fruit in the case of a self-fertilised plant was 

 0-009 gr., from geitonogamy 0*012 gr., and on cross-fertilisation 

 0'013 gr. 



On the whole, however, the results of geitonogamy show that the 

 favourable effects of cross-fertilisation do not depend simply on the 

 fact that the pollen of one flower is conveyed to the stigma of another. 

 But the plants which are crossed must in some way be different. If 

 plants of Ipomoea purpurea (and Mimulus luteus) which have been 

 self-fertilised for seven generations and grown under the same con- 

 ditions of cultivation are crossed together, the plants so crossed 

 would not be superior to the self-fertilised; on the other hand 

 crossing with a fresh stock at once proves very advantageous. The 

 favourable effect of crossing is only apparent, therefore, if the parent 

 plants are grown under different conditions or if they belong to 



1 Similarly crossing in the case of flowers of Pelargonium zonale, which belong to plants 

 raised from cuttings from the same parent, shows no superiority over self-fertilisation. 



2 Cross and Self fertilisation (1st edit.), p. 444. 



