FLOWERS AND INSECTS 
125 
in solving these problems. They often fail, but succeed 
often enough to make the effort worth while. 
89. Preventing self-pollination. It is evident that this 
danger arises only in those flowers in which the stamens 
and carpels are associ- 
ated,, but their separa- ^ " ^ 2 
tion in different flowers 
may be considered as 
one method of prevent- 
ing self-pollination. In 
order to understand the 
various arrangements to 
be considered, it is nec- 
essary to explain that 
the carpel does not re- 
ceive the pollen indif- 
ferently over its whole 
surface. There is one 
definite region organ- 
ized, known as the 
stigma, upon which the 
pollen must be deposited 
if it is to do its work. 
Usually this is at the 
most projecting point 
of the carpel, very often 
at the end of a stalk- 
like prolongation from 
the ovary (the bulbous 
part of the carpel), 
known as the style; 
sometimes it may run down one side of the style. When 
the stigma is ready to receive pollen it has upon it a 
sweetish, sticky fluid, which holds and feeds the pollen. 
In this condition the stigma is said to be mature ; and the 
pollen is mature when it is shedding, that is, ready to fall 
FIG. 131. Parts of the flower of rose acacia 
(Robiniahispida). In 1 the keel is shown pro- 
jecting from the hairy calyx, the other more 
showy parts of the corolla having been re- 
moved. Within the keel are the stamens 
and the carpel, as seen in 3. The keel forms 
the natural landing place of a visiting bee, 
whose weight depresses the keel and causes 
the tip of the style to protrude, as shown in 
2. This style tip bears pollen upon it, 
caught among the hairs, seen in 3, and as it 
strikes the body of the bee some pollen is 
brushed off. If the bee has previously visited 
another flower and received some pollen, it 
will be seen that the stigma, at the very tip 
of the style, striking the body first, will very 
probably receive some of it. The nectar pit 
is shown in 3, at the base of the uppermost 
stamen. After GRAY. 
