FERTILIZATION. 



169 



In perfect flowers self-fertilization would commonly occur 

 unless it were prevented by the action of the essential organs 

 or by something in the structure of the flower. In reality 

 flowers which at first sight would appear to be designed to 

 secure self-fertilization are almost or quite incapable of it. 

 Frequently the pollen from another plant prevails over that 

 which the flower may shed on 

 its own pistil, so that when 

 both kinds are placed on the 

 stigma at the same time it is 

 the foreign pollen which causes 

 fertilization. But apart from 

 this fact, there are several 

 means of insuring the pres- 

 ence of foreign pollen, and 

 only that, upon the stigma, 

 just when it is mature enough 

 to receive pollen tubes. 



208. Stamens and Pi.'itils 

 maturing at Different Times. 

 — If the stamens mature at 

 a different time from the 

 pistils, self-fertilization is as 

 eif ectually prevented as though 

 the plant were dioecious. This 

 unequal maturing or dichog- 

 amy occurs in many kinds of 

 flowers. In some, the figwort 

 and the common plantain, for example, the pistil develops before 

 the stamens, but usually the reverse is the case. The Clero- 

 dendron, a tropical African flower, illustrates in a most strik- 

 ing way the development of stamens before the pistil. The 

 insect-visitor, on its way to the nectary, can hardly fail to 

 brush against the protruding stamens of the flower in its 



Fig. 152. — Flower of Clerodendron in 

 Two Stages. 



In the upper figure (earlier stage; the 

 stamens are mature, while the pistil is 

 still undeveloped and bent to one side. 

 In the lower iigure (later stage) the 

 stamens have withered and the stigmas 

 have separated, ready for the reception 

 of pollen. 



