^ 



THE POLLEN GRAIN AND ITS WORK 41 



sac, egg, and primary endosperm nucleus. Redraw this figure 

 on a sheet of paper and label the parts. 



Although pistils vary much in number of carpels, length of 

 styles, and in number of locules and ovules, there is uniformity 

 in organization and adaptation of parts to special functions. 

 The stigma is especially adapted for receiving pollen, the style 

 supports the stigma in a position suitable for receiving the pollen, 

 and the ovary protects the delicate ovules in which is the embryo 

 sac containing the egg and primary 

 endosperm nucleus, which are the 

 chief structures of the pistil. 



The Stamen. — The stamen usu- 

 ally consists of two parts; the en- 

 larged terminal portion, or anther; 

 and the stalk, or filament. The 

 filament is often so short as to seem 

 to be absent. Point out the parts 



of the stamen in A of Figure 48. 



rrir j.u • n r i i, i FiG. 48. — A, Stamen, a, an- 



The anther is usually four lobed .u f a^ \ t> u 



. Ill- • ther; /, filament. B, much en- 



and Wlthm each lobe is a cavity, larged cross section of an anther, 

 called locule, which contains many showing the locules and pollen 

 globular bodies known as pollen or grains. The two locules at the 

 pollen grains. When the pollen is ^''^{igj^''7gg°^'J|;'''^' ^"^^^^^ ^^'^ 

 mature, the walls of the . anther ^° ^" escape, 

 open and allow the pollen to escape. Notice the cross sec- 

 tion of an anther shown in B of Figure J/S- Point out the 

 locules and pollen grains. Notice that two of the locules have 

 opened. 



The Pollen Grain and its Work. — The pollen grain is a cell 

 with its living matter enclosed in a heavy protective wall. It 

 needs to be well protected, for during its journey to the pistil, 

 destructive agencies such as cold, heat, and drying are encoun- 

 tered. The transference of the pollen to the stigma is called 

 pollination. Pollination is a very important event, for the pollen 

 cannot perform its function except on the stigma. 



On the stigma the pollen grain grows a tube which traverses 

 the stigma and style, pierces the ovule, and reaches the embryo 

 sac. Pollen grains, when first formed in the anther, have only 

 one nucleus, but in preparation for the work of fertilization, th(M-e 

 is nuclear division and as a result there are three nuclei in a well 



