I.] IMPORTANCE OF CROSS-FERTILISATION. 7 



it is so has been clearly proved. Kolreuter speaks 

 with astonishment of the " statura portentosa" of some 

 plants thus raised by him ; indeed, says Mr. Darwin 

 ("Animals and Plants under Domestication," ch. xvii.), 

 " all experimenters have been struck with the won- 

 derful vigour, height, size, tenacity of life, precocity, 

 and hardiness of their hybrid productions." Mr. 

 Darwin himself, however, was, I believe, the first to 

 show that if a flower be fertilised by pollen from a 

 different plant, the seedlings so produced are much 

 stronger than if the plant be fertilised by its own 

 pollen. I have had the advantage of seeing several 

 of these experiments, and the difference is certainly 

 most striking. For instance, six crossed and six self- 

 fertilised seeds of Ipomcea purpurea were grown in 

 pairs on opposite sides of the same pots ; the former 

 reached a height of 7 ft., while the others were on 

 an average only 5 ft. 4 in. The first also flowered 

 more profusely. It is moreover remarkable that in 

 many cases plants are themselves more fertile if sup- 

 plied with pollen from a different flower, a different 

 variety, or even, as it would appear in some instances 

 (in the passion flower, for instance), from a different 

 species. Nay, in some cases pollen has no effect 

 whatever unless transferred to a different flower. 

 Fritz Miiller has recorded some species in which 

 pollen, if placed on the stigma of the same flower, 

 has not only no more effect than so much inorganic 

 dust ; but, which is perhaps even more extraordinary, 

 in others, he states that the pollen placed on the 

 stigma of its own flower acted on it like a poison. 

 This he noticed in several species: the flower faded and 



