218 AGRICULTURE. 



generations. Examples of this tendency to 

 vary are seen in the grape and the potato. 



Another condition is (2) the time required for 

 the growth of a plant from the seed to the ma- 

 turity of its seed. Many species require but a 



FIG. 55. ROOTED TIPS OK A SEEDLING RASPBERRY CANE. 



They are of one season's growth, showing new plants formed and their root- 

 systems. (From Normal Garden.) 



single season ; most trees require a number of 

 years to produce one generation. Four years 

 from seed to seed would be a short period for 

 apple, pear, and cherry, and some varieties re- 

 quire a much longer time. 



Hence, it would be impracticable, if not im- 

 possible, in the lifetime of one man, to render 



