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 Pistils 
maturing at Different Times. 
—If the stamens mature at | 
a different time from the yg. 152,— Flower of Clerodendron in 
pistils, self-fertilization is as Two Stages. 
In the upper figure (earlier stage) the 
effectually prevented as though stamens are mature, while the pistil is 
the plant were dicecious. This still undeveloped and bent to one side. 
In the lower figure (later stage) the 
unequal maturing or dichog- stamens have withered and the stigmas 
amy occurs in many kinds of have separated, ready for the reception 
flowers. In some, the figwort gal cepa . 
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 
