212 BOTANY: PRINCIPLES AND PROBLEMS 



knew the exact ancestry of every individual plant and the 

 characters displayed by each of its ancestors and descendants. 

 This method involves much care and pains, both in making 

 artificially the particular pollinations desired and in preventing 

 all pollinations by such uncontrolled agencies as insects and the 

 wind, and the labor of keeping the records is often very great. 



Fig. 122. — Gregor Johann Mendel, 1822-1884. (From Genetics, by jicrmission) . 



Mendel's method is now almost universally adopted, however, 

 by students of inheritance. Third, in each generation where 

 contrasting characters appeared (both red flowers and white ones 

 in the offspring from a single cross, let us say) he carefully counted 

 the number of individuals of each type and thus obtained a 

 mathematical statement of the facts. In short, Mendel applied 

 the true experimental method to the problems of heredity. 



The results derived by this novel and painstaking method 

 of investigation were carefully reported by Mendel and his inter- 

 pretations thereof have come to be known as Mendel's Law. This 

 law, however, is not a single proposition but really a series of 

 distinct principles. Its important points we shall now briefly 

 discuss. 



Unit Characters. — ^As a general result of his hybridization 

 experiments, Mendel observed that the plant seems to behave in 



