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 necr 

 essary to explain that 

 the carpel does not re- 

 ceive the pollen indif- 

 ferently over its whole 



There is one 

 region organ - 



surface, 

 definite 



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. AVhen 

 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 being shed, 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 receivetl 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. 



