CHAPTER XII. POLLINATION AND FERTILIZATION. 



95 



But even in cases where the flowers are capable of self-fertil- 

 ization, the pollen from other plants is commonly more effective 

 than their own ; and, as showy or nectar-bearing flowers are 

 almost constantly visited by insects, the chances are that cross- 

 fertilization will usually be effected. 



(4) By heteromorphism, or by the existence within the limits 

 of the same species of flowers of different kinds. In some cases 

 there are two different kinds of flowers, — one with short stamens 

 and long styles, and the other with long stamens and short syles. 



Fig. 287 



Fig. 287. — a, two long-styled flowers of Mitchella repens. b, the tubular corolla of one 

 laid open so as to show the stamens. 



Fig. 288. — a, two short-styled flowers of same species ; the styles are concealed in 

 the tube of the corolla, whiie the stamens protrude, b, one of the corolla-tubes laid open to 

 show the stigmas. 



Such flowers are called dimorphous. Figs. 287 and 288 represent, 

 respectively, the long-styled and short-styled flowers of Mitchella 

 repens. The flowers occur in pairs, usually grown together, 

 more or less, at the base, as shown in the figures a, a. It will be 

 seen that the stigmas of the left-hand pair stand at about the 

 same level as the anthers of the right-hand one. The two different 

 kinds of flowers invariably occur on different plants of the same 

 species. The lower figures, b, b, represent a flower of each kind, 



