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 
Fic. 52. — A, diagrammatic section 
of an ovule of the Tomato in which 
the egg (b) and primary endosperm 
nucleus (d) have been fertilized. 0, 
portion of ovule surrounding and en- 
closing the embryo sac. B, diagram- 
matic section of the seed of the 
Tomato. e, embryo; c, endosperm; 
1, seed coat. The lines 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 pollen grains grow different parts of the seed have de- 
their tubes into the stigma. p, veloped. Both are enlarged but the 
pollen grains; ¢, pollentube. Much _ ovule is enlarged much more than the 
enlarged. 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 aseed. There 
