232 PRACTICAL COURSE IN BOTANY 



the market with ahnost as great frequency and in as ready- 

 response to demand as the new styles of women's bonnets 

 and gowns in the shop windows. 



263. Causes of variation. — While man cannot directly 

 force plants to vary in any given direction, he can hasten the 

 process of variation by crossing, or by changing the conditions 

 under which they are growing. This is called " breaking 

 the type." Hybridization furnishes the readiest means to 

 this end. Change of food supply, especially if accompanied 

 by excess of nourishment, is probably the expedient that 

 ranks next in effectiveness. Light, temperature, moisture, 



Fig. 339. — Variation in blackberry leaves due to hybridization. 



character of the soil, exposure to wind, and the like, also 

 have their influence; and in adapting themselves to changes 

 in these various conditions, plants are apt to exhibit an 

 unusual number of variations, when removed from one local- 

 ity to another, especially if the difference in soil and climate 

 is very marked. Now comes the breeder's opportunity. By 

 taking advantage of such variations as may occur either 

 spontaneously, or as the result of his efforts to break the type, 

 he will generally find some that will meet his requirements; 

 and knowing the effect produced by different conditions, he 

 can, to a certain extent, influence the course of variation in 

 the direction desired, by subjecting his specimens to the 



