POLLINATION AND FERTILIZATION 153 
of the most effectual means of preventing self-pollination in 
bisexual flowers is the maturing of the stamens at a different 
time from the pistils; this is known as dichogamy. In some 
flowers, as in the figwort and some 
plantains (fig. 136), the pistils ma- 
ture first. In such cases the pollen 
from older flowers (in the stami- 
nate condition) is transferred to the 
stigmas of recently opened flowers 
(in the pistillate condition). 
At the left, earlier stage with Usually, as in some mallows 
stamens mature but the stigmas and in the gentians and fireweeds 
esa at Aap eg Soe (Epilobium) (fig. 137), the stamens 
stamens withering but the stig mature first. An insect visitor to 
mas spread apart and ready for 4 newly opened flower in the stami- 
pollination : tas 
nate condition becomes somewhat 
covered with pollen. Then, flying to an older flower in the 
pistillate condition, the insect is almost sure to leave pollen 
on the stigmas and thus insure cross-pollination. 
It is common to find the stamens of a flower maturing a 
few at a time, as in “ nasturtium,” buckwheat, and many other 
flowers. This gives 
more opportunities 
for insects to carry 
away the pollen 
than would be pos- 
sible if it all ma- 
tured at once. 
145. Prevention 
of self-pollination ; 
dimorphism. A 4, long-styled form; B, short-styled form ; a, anthers; 
means of prevent- s, stigmas. About 3 times natural size 
ing self-pollination 
which is even more effective than dichogamy is found in the 
structure of flowers in which some have a long pistil and short 
stamens, others a short pistil and long stamens. This condition 
Fre. 137. Dichogamous flowers 
of fireweed (Epilobium) 
Fic. 188. Lengthwise section of dimorphous 
flower of bluets 
