THE FAMILY OF GRASSES 343 



why they get few varieties in the field grains and 

 so many in corn and singly cultivated garden 

 vegetables, in which variation is quite evident and 

 varieties are easily segregated. 



It is obviously necessary that a plant grown 

 from the seed, and for its seed, must reproduce 

 itself accurately from generation to generation; 

 otherwise the agriculturist could have no assur- 

 ance as to what might come forth when he sows 

 his grain. 



In fact, the numerous varieties have become 

 fixed so that each may be sown with a large 

 measure of assurance that the crop will have the 

 uniform character of the seed. The differences 

 among the different varieties have to do with 

 size of grain, productivity, season of ripening, 

 protein content, quality of so-called hardness, 

 which is important in bread-making, color of 

 grain, peculiarities as to beards, chaff, and the 

 like; and perhaps most notable of all condi- 

 tion of susceptibility or immunity to the attacks 

 of the fungus known as rust, which is the chief 

 enemy of the wheat, and a perpetual menace to 



the crop. 



A MICROSCOPIC PEST 



There are always some compensations associa- 

 ted with any specialized development in a culti- 

 vated plant or a domesticated animal. 



