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- 



12 



