UNIT CHARACTERS 177 
be present, though one may be hidden. What will 
happen in the grandchildren ? 
The manner in which characters comparable with 
the above are actually transmitted has been worked 
out in the case of many races of animals and plants, 
and in cases where experimental matings can be 
readily carried out, and a large number of offspring 
reared, it is found that a simple rule applies which 
holds good in every example thoroughly examined 
hitherto. This law was discovered by Mendel about 
the year 1865, and has since been called by his name. 
Before enunciating it we shall consider the informa- 
tion afforded by the case of a single pair of simple 
characters. Afterwards we shall endeavour to show 
the application of the law to the more complex cases 
in which combinations of characters are concerned. 
A grain of Indian corn or maize contains a germ or 
embryo, which under suitable conditions will give rise 
to the future plant. The embryo is surrounded by a 
certain amount of reserve food material constituting 
the endosperm—a store which is made use of by the 
young plant during its germination. The embryo 
arises as the result of a process of fertilization which 
takes place in the following manner: The ovum, or 
female cell hidden in a flower, contains a nucleus, and 
this on fusion with one of the nuclei derived from a 
grain of pollen initiates the vital processes which lead 
to the development of an embryo plant. 
Nuclei are the central and, from the point of view of 
heredity, the most important parts of cells—the con- 
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