ECOLOGY OF FLOWERS; POLLINATION 175 
monecious flowers may be pollinated by staminate ones 
on the same plant, but this does not secure as good seed 
as 1s secured by having pollen brought to the pistil from 
a different plant of the same kind. 
In perfect flowers self-pollination 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 many flowers which at first sight would appear to 
be designed to secure self- 
pollination are almost or 
quite incapable of it. Fre- 
quently 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 Fic. 131. Flower of Clerodendron in 
Two Stages. 
are several means of insur- 
ing the presence of foreign In 4 (earlier stage) the stamens are ma- 
ture, while the pistil is still undevel- 
pollen, and only that, upon oped and bent to one side. In B (later 
the stigma, just when it is stage) the stamens have withered and 
: the stigmas have separated, ready for 
mature enough to recelve the reception of pollen. 
pollen tubes. 
209. Stamens and Pistils maturing at Different Times. 
If the stamens mature at a different time from the pistils, 
self-pollination is as effectually prevented as though the 
plant were diecious. This unequal maturing or dichogamy 
