178 MENDELISM 
stituent units of the plant body. The cells which, 
together with their nuclei, take part in the process of 
fertilization are known as gametes, or germ-cells—male 
and female respectively, the latter being the ovum. 
It is less generally known that the endosperm of a 
grain of Indian corn arises by a very similar process 
to the one which gives rise to the embryo itself. A 
second nucleus derived from the same pollen grain 
fuses with a nucleus situated near the ovum, and to 
the product of this fusion the endosperm owes its 
origin. It is further found, so far at least as those 
characters are concerned to which we shall at present 
confine our attention, that these two important nuclei 
hidden in the same female flower are exactly alike in 
hereditary constitution, and so are the two generative 
nuclei derived from a single pollen grain. In conse- 
quence of this fact, the observed character of the endo- 
sperm may be regarded as a true guide to the nature 
of the plant into which the associated embryo will 
afterwards develop. The hereditary qualities of the 
two are exactly the same. 
It is not difficult to find a variety of Indian corn in 
which the endosperm is yellow, and another in which 
the colour of this tissue is white, owing to the absence 
of any visible yellow pigment. If a female flower of a 
white variety is fertilized with pollen taken from a 
yellow variety, the resulting grain shows its hybrid 
nature by the presence of the yellow colour in its endo- 
sperm. This is found to be a regular rule. Grains 
upon a plant of a white strain which has been pollinated 
with ‘white pollen’ are white, but if pollinated from 
