232 PRACTICAL COURSE IN BOTANY 
the market with almost 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, 
Fic. 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 
