PLANT BREEDING 115 
167. The breeding of plants is an application of the 
principles of reproduction and heredity to the production 
of plants with certain desirable characteristics. In- 
stead of waiting for the chance production of a desirable 
type of plant, the plant breeder either grows many plants 
in conditions under his control and selects for further 
propagation those he deems most desirable (method of 
selection), or he takes two distinct plants, each with 
certain characters that he desires, and crosses them, and 
grows the progeny in large numbers for several generations 
until by the laws of chance in the distribution of the 
unit character determinants there appears a plant 
combining the desirable characters of the two parents. 
This is the method of hybridization or crossing. The 
discovery by Mendel of the segregation of characters by 
definite laws of numbers (see paragraph 165) has given a 
great impetus to this line of work. 
Laboratory Studies. Not much can be done in the way of 
laboratory work on this subject. In the study of the different 
forms of plants in the later chapters of the book, the points 
emphasized in the foregoing paragraphs should be borne in 
mind. A few suggestions are made for observations on the 
part of the student. 
(a) Find and compare carefully a dozen different plants of 
timothy (Phleum pratense), red clover (Trifolium pratense), 
ribbed plantain (Plantago lanceolata), ete. Select those 
plants of the same age and from as nearly as possible the same 
soil and growing under the same environmental conditions. 
Note how they differ in height ; number, size and shape of leaves; 
size of flower heads; number of flowers in the head; amount of 
hairiness of various parts, etc. 
(6) Compare plants of the same kind grown in sun and shade, 
in dry and moist soils, in barren and on fertile ground, for 
differences due largely to the environment. Note the differ- 
ences in the times of flowering and of ripening of seeds, as well 
as the structural differences. 
