THE FAMILY OF GRASSES 347 



by saving for seeding purposes the grain of 

 plants that proved individually resistant to the 

 rust, have long been carried out more or less 

 systematically. 



Partly in this way, and partly perhaps through 

 accidental development in regions where the rust 

 does not prevail, some varieties of wheat have 

 been introduced that show a large measure of 

 immunity to the disease. But unfortunately 

 these for the most part have been varieties that 

 did not produce grain of very good quality. In 

 general the favorite wheats of the world have 

 remained subject to the attacks of the fungus. 

 Their degree of immunity in any given season 

 has depended upon accidental conditions of 

 weather that interfered with the development 

 or spread of spores of the rust fungus rather 

 than upon any inherent resistance of the cereal 

 itself. 



Thus it is familiar experience everywhere that 

 the farmer cannot have any full assurance as to 

 the amount of his grain crop until the grain 

 approaches the ripening stage; because at any 

 time the invisible spores of the rust may sweep 

 as a devastating horde across his fields and, find- 

 ing lodgment on the grain stalks, so devitalize 

 them as greatly to reduce their capacity for seed 

 formation. 



