ECOLOGY OF FLOWERS 363 
incapable of it. Frequently the pollen from another plant 
of the same species 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 presence of 
foreign pollen, and only that, upon the stigma, just when 
it is mature enough to 
receive pollen tubes. 
435. Stamens and Pistils 
maturing at Different 
Times. —If the stamens 
mature at a different time 
from the pistils, self-polli- 
nation is as effectually pre- 
vented as though the plant 
were dicecious. This un- 
equal maturing or dichog- 
amy occurs in many kinds 
of flowers. In some, the Fie. tak (ikekee | Ee 
figwort and the common In A (earlier stage) the stamens are mature, 
plantain for example, the vhletnenietlisat mderalonsd andes 
pistil develops before the have withered and the stigmas have sepa- 
stamens, but usually the rated, ready for the reception of pollen. 
reverse is the case. The Clerodendron,} a tropical African 
flower (Fig. 255), illustrates in a most striking 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 earlier stage 
(at A), but it cannot deposit any pollen on the stigmas, 
1C, Thompsoniz. 
