120 MENDELISM CHAP. 



grounds for assuming that anything in the nature of 

 unproductive fertilisation takes place. 1 



If we turn from animals to plants we find a more com- 

 plicated state of affairs. Generally speaking, the higher 

 plants are hermaphrodite, both ovules and pollen grains 

 occurring on the same flower. Some plants, however, 

 like most animals, are of separate sexes, a single plant 

 bearing only male or female flowers. In other plants the 

 separate flowers are either male or female, though both 

 are borne on the same individual. In others, again, the 

 conditions are even more complex, for the same plant 

 may bear flowers of three kinds, viz. male, female, and 

 hermaphrodite. Or it may be that these three forms 

 occur in the same species but in different individuals 

 -female and hermaphrodites in one species; males, 

 females, and hermaphrodites in another. One case, 

 however, must be mentioned as it suggests a possibility 

 which we have not hitherto encountered. In the com- 

 mon English bryony (Bryonia dioica} the sexes are sep- 

 arate, some plants having only male and others only fe- 

 male flowers. In another European species, B. alba, both 

 male and female flowers occur on the same plant. 

 Correns crossed these two species reciprocally, and also 

 fertilised B. dioica by its own male with the following 

 results : 



1 For the most recent discussion of this peculiar case the reader is re- 

 ferred to Professor Castle's paper in Science, December 16, 1910. 



