II.] MODES OF CROSS-FERTILISATION. 29 



pollen from the anther of the one form to the pistil of 

 the other. These plants are termed heteromorphous ; 

 some of them have two kinds of flowers, and are 

 called dimorphous ; while others have three forms, and 

 are called trimorphous. 



But even in plants which belong to none of these 

 categories we find minor modifications which tend 

 to prevent self-fertilization ; and Mr. Darwin is pro- 

 bably right in his opinion that no plant invariably 

 fertilizes itself. Thus in some species where the 

 stamens surround the pistil, and which might, there- 

 fore, be supposed to be arranged in such a man- 

 ner as to ensure self-fertilization, the anthers do not 

 open towards the pistil, but on the sides, and by no 

 means therefore in a favourable position with reference 

 to the transference of the pollen. In most, if not all 

 the Cruciferae, the anthers in young flowers have the 

 side which opens turned towards the pistil ; but be- 

 fore the anthers come to maturity they twist them- 

 selves round, so as to turn their backs to the stigma. 

 Again, in pendent flowers, where the pistil hangs 

 below the anthers, the stigmatic surface is never the 

 upper one, which would catch any falling pollen ; but 

 on the contrary, the lower one, which could hardly be 

 touched by the pollen of the same flower, but which 

 is so placed as to come in contact with any insect 

 or other body approaching the flower from below. 



It is also probable that many cases will be found to 

 exist, in which, though the pollen necessarily comes in 

 contact with the pistil of the same plant, fertilization 

 does not take place. However improbable this might 

 ^priori appear, it is nevertheless said by Hildebrand 



