200 



GROWTH AND WORK OF PLANTS 



the larger mass containing the other nucleus, as shown in fig. 



150. Very soon this smaller cell becomes free and floats in the 



protoplasm of the larger cell. This is the condition in which 



most ripe pollen grains are; there are 

 two cells, a small one floating within 

 the protoplasm of the larger one. The 

 larger cell is sometimes called the tube 

 cell because later it grows out to form 

 the pollen tube. The smaller one float- 

 ing within the larger one is called the 

 generative or body cell.* After a pollen 

 grain falls on the stigma it germinates 

 and forms a long tube which grows 

 down through the style into the ovary, 

 where it enters the ovule. The two 



nuclei move into the pollen tube. The nucleus of the body cell 



divides into two nuclei either in the tube or before entering it. 



These two nuclei are the sperm nuclei, or male nuclei, and they are 



brought into the ovule by the pollen tube. 



Fig. 151. 



Germinating pollen 

 grains of Peltandra; 

 generative nucleus in 

 one undivided, in other 

 divided to form the two 

 sperm nuclei; vegetative 

 nucleus in each near 

 the pollen grain. 



316. Structure of the ovule. The ovule is nearly oval in 

 form. There are usually two coats on the ovule, but sometimes 

 only one. Except at the stalk end where the tissues are more or 

 * Or central cell of the antheridium. See Chapter XXXVII. 



