THE DEVELOPMENT OF THE OVULE INTO A SEED 43 



fusions, there are two fertilizations, and the two fertilizations 

 are called " double fertilization." Both egg and primary endo- 

 sperm nucleus are now said to be fertilized, and the poUen grain, 

 has performed its function, which is an important one, for with- 

 out fertilization the ovule would not develop into a seed. 

 Pollination, the growth of the pollen tube to the embryo sac, 

 and the formation of the two sperms are simply preliminary 

 acts to fertilization, which is the final achievement of the pollen 



grain. Study the pollen grains 

 shown in Figure Jfi. Notice that 

 the tube has broken through the 



Fig. 52. — A, diagrammatic section 

 of an ovule of the Tomato in whicli 

 the egg (6) and primary endosperm 

 nucleus (d) have been fertilized, o, 

 portion of ovule surroimding and en- 

 closing the embryo sac. B, diagram- 

 matic section of the seed of the 

 Tomato, e, embryo; c, endosperm; 

 t, seed coat. The Unes drawn from 

 the ovule to the seed indicate the 

 Fig 51. — Stigma of Corn show- parts of the ovule from which the 

 ing how the poUen grains grow different parts of the seed have de- 

 their tubes into the stigma, p, veloped. . Both are enlarged but the 

 poUen grains; t, pollen tube. Much ovule is enlarged much more than the 

 enlarged. seed. 



pollen waU. How have the two sperms been formed? In Figure 

 50 trace the pollen tubes to the embryo sac. How do the pollen 

 tubes make their way through the style? Where do they obtam 

 their food for growth? Notice how the pollen tubes enter the 

 branched stigma of Corn in Figure 51. 



The Development of the Ovule into a Seed. — After the egg and 

 primary endosperm nucleus have been fertihzed, the ovule begins 

 its development, which results in the production of a seed. There 



