FERTILIZATION AND DEVELOPMENT OF SEED 183 



in fig. 141. Very soon this smaller cell becomes free and its pro- 

 toplasm floats in the protoplasm of the larger cell. This is the 

 condition in which most ripe pollen grains are; there are two cells, 

 one floating within the protoplasm of 

 the larger one. The larger cell is 

 sometimes called the tube cell because 

 later it grows out into the pollen tube. 

 The smaller one floating 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 grain- 

 body cells divide into two nuclei either in the tube or before enter- 

 ing it. These two nuclei are the sperm nuclei, or male nuclei, and 

 they are brought into the ovule by the pollen tube. 



301. 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 rr 



Fig. 143. 



A represents a straight (orthotropus) ovule of Polygonum; B, the inverted (anatropus) ovule 

 of the lily; and C, the right-angled (campylptropus) ovule of the bean; /, funicle; c, chalaza; 

 k, nucellus; ai, outer integument; ii, inner integument; m, micropyle; em, embryo sac. 



less blended, the coats of the ovule can be seen in a lengthwise 

 section as two distinct layers of tissue, but there is a circular 



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



