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 pollen 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 49. Notice that 

 the tube has broken through the 



FIG. 51. Stigma of Corn show- 

 ing how the pollen grains grow 

 their tubes into the stigma, p, 

 pollen grains; t, pollen tube. Much 

 enlarged. 



FIG. 52. A, diagrammatic section 

 of an ovule of the Tomato in which 

 the egg (6) and primary endosperm 

 nucleus (d) have been fertilized, o, 

 portion of ovule surrounding and en- 

 closing the embryo sac. B, diagram- 

 matic section of the seed of the 

 Tomato, e, embryo; c, endosperm; 

 t, seed coat. The lines drawn from 

 the ovule to the seed indicate the 

 parts of the ovule from which the 

 different parts of the seed have de- 

 veloped. Both are enlarged but the 

 ovule is enlarged much more than the 

 seed. 



pollen wall. 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 obtain 

 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 fertilized, the ovule begins 

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



