158 CROSS-FERTILISATION. [CH. XII 



their pollen in a minute explosion. The same thing is 

 seen in the " artillery plant " (Pilea) which is sometimes 

 grown in green-houses, and receives its name from the 

 puff of smoke-like pollen given out from its exploding 

 stamens. 



Wind-fertilised plants also show a certain resemblance 

 to one another in the character of the gyncecium. Since 

 the stigma receives the pollen fortuitously, the chance of 

 fertilisation is increased when the stigma is large. The 

 stigmatic surface in the plantain is great in proportion to 

 the size of the flower, and the same thing is particularly 

 striking in the wheat-flower figured above (fig. 71). This 

 is a general character of the class of flowers we are 

 considering, though the extension of surface is brought 

 out in different ways, for instance in the walnut the 

 stigma is a broad plate-like structure, instead of being 

 papillated or divided, as in the plantain and the grass. 



Self- and cross-fertilisation. 



When a flower is fertilised by pollen from its own 

 anthers or from the anthers of a flower on the same plant, 

 the process is called self-fertilisation. When the pollen 

 comes from a distinct individual, it is known as cross- 

 fertilisation. Some species of plants, for instance the 

 nettle, are divided into two classes of individuals: (1) 

 consisting of plants whose flowers have stamens but no 

 carpels ; (2) of plants whose flowers have carpels but no 

 stamens. Reproduction must in this case (if it occurs at 

 all) be the result of cross-fertilisation. But in the flowers 

 whose structure we have been considering it is obvious 



