VEGETATIVE AND SEXUAL REPRODUCTION 163 



pollen sacs gape open, thus enabling the pollen to escape. The 

 pollen is then free to sift out and to be deposited by the wind 

 or by insects upon the stigma of the same or a different flower. 

 The deposit of pollen on the stigma is termed pollination and is 

 essential to fertilization and the setting of seed. 



The pistil may be borne singly on the receptacle, as in the 

 mandrake, or there may be a cluster of separate pistils in a single 

 flower, as in the buttercup. The pistil is composed of a sac, or 

 flask-shaped lower portion, called the ovary, and of a terminal 

 portion, called the stigma. The stigma is usually roughened, 

 irregular, or furnished with hairs or a sticky fluid for the 

 retention of pollen brought to it during pollination. In many 

 cases the stigma is joined to the ovary by a narrow neck called 

 the style. The style is sometimes lacking, and then the stigma 

 is said to be sessile. The ovary bears the ovules on a cellular out- 

 growth, or ridge, called the placenta. The ovules (Fig. 84, , <?) 

 develop into the seeds after fertilization (Fig. 86, c, <f). 



GAMETOGENESIS, FERTILIZATION, AND DEVELOPMENT 



Gametes, or sex cells. The general facts concerning the 

 structure of the sex cells, or gametes, and their union in fertili- 

 zation have already been discussed under the head of sexual 

 reproduction. It remains, therefore, to give more in detail the 

 origin and development of the sex cells within the pollen grain 

 and ovule, and to explain more fully how fertilization is effected 

 within the ovule through the agency of the pollen tube. 



In describing the formation of the gametes it will be neces- 

 sary to include the structure of pollen and ovules, in order to 

 make clear the processes by which the gametes are developed 

 within these structures. 



The young pollen grain is a single cell with a thick cell wall 

 inclosing a dense granular protoplast with a large nucleus 

 (Fig. 85, a). The thickened cell wall, which is designed to 

 protect the pollen grain against desiccation, is divided into a 

 thick outer layer and a thin inner layer. The outer thickened 

 layer is protective, and the inner layer is important in the 



