THE EGGS 
The eggs are located in embryo sacs within the pistils on the young 
cob. The pistil is made up of the silk (stigma) and a basal portion, 
the ovary, the wall of which after the fertilization of the egg becomes 
modified to form the outer layer (pericarp) of the mature kernel, 
(pi. 3, B.) The embryo sac is a specialized structure within the 
pistil and contains one egg, other cells, and two polar nuclei. All of 
these elements of the embryo sac originate from a single cell, the 
megaspore, and are qualitatively alike. Thus there is a pistil for 
every mature kernel that is produced, and within each pistil a single 
egg and two polar nuclei, the fertilization of which initiates the 
development of the kernel. 
THE POLLEN 
The sperms are formed within the pollen grains which are pro- 
duced in the tassel. Each mature pollen grain contains two sperms. 
A normal corn plant produces many millions of pollen grains, and 
as it rarely produces more than 1,500 kernels there is a large excess of 
pollen. 
POLLINATION 
As a plant reaches sexual maturity the tassel begins to shed its 
pollen and the silks emerge from the husks. Pollination consists 
in the transfer of pollen grains from the tassels to the exposed silks 
of ear shoots. The silks are covered with minute hairs which, with 
a slightly sticky substance that is exuded at this time, assist in 
holding the pollen grains. Because of the spreading of the pollen 
by the wind, probably 95 per cent or more of the silks receive pollen 
from plants other than those on which the silks grow, or in other 
words are cross-pollinated. 
FERTILIZATION AND KERNEL DEVELOPMENT 
The pollen grain germinates soon after it becomes lodged on the 
exposed silk of the ear shoot. A pollen tube develops rapidly and 
enters the silk. It then grows down through the silk, and finally 
into the embryo sac, where the end is ruptured. The two sperms 
reach the embryo sac by means of the pollen tube. One of the 
sperms unites with the egg, and the other unites with the two polar 
nuclei. The embryo or germ develops by repeated divisions of the 
fertilized egg, and the endosperm is produced by the division of the 
nucleus formed by the fusion of the second sperm with the two polar 
nuclei. The endosperm constitutes all of the kernel except the em- 
bryo and the pericarp. The pericarp is developed from the ovary 
wall and is composed wholly of tissue of maternal origin. The 
different parts of the kernel are indicated diagrammatically in 
Figure 2. 
The cells of the corn embryo normally contain 20 chromosomes, 10 
of which were contributed by the sperm nucleus and 10 by the egg 
cell. The endosperm cells contain 30 chromosomes, 10 of which were 
contributed by the sperm nucleus and 20 by the polar nuclei. These 
