THE WHEAT RUST 67 



spore is small, consisting of (usually) four cells. From 

 each cell of this small plant a short branch grows ; the outer 

 end of the branch swells, is cut off by a wall, and becomes 

 a spore of a fourth kind which is called a sporidium. A rust 

 sporidium, carried to a leaf, young stem, or flower of a bar- 

 berry, may germinate there. The germ tube makes its way 

 directly through a surface cell (not as a rule, at least, through 

 an air-pore) and so infects the barberry. 



96. Outline of the History of the Wheat Rust. We see 

 that the complete life story of the wheat rust includes three 

 different plants, which are really so many distinct generations. 

 These three plants produce four different kinds of spores 

 (or, if we include the spermatia, five spore-like structures). 

 The plant living on the barberry bears spring spores (and 

 spermatia) ; the plant living on the wheat bears summer 

 spores and winter spores ; and the small plant developed 

 from a winter spore bears sporidia. 



97. Damage Done by the Wheat Rust. The wheat plant 

 is weakened because the food stored in its cells for its own use 

 is taken by the rust. An infected plant does not grow to its 

 normal size, its grains are small and shriveled, and so the 

 wheat crop is injured both in quality and in quantity. The 

 rust occurs in all parts of the world where wheat is grown, 

 and in some regions the cultivation of wheat has been given 

 up because of the prevalence of the disease. It is worst in 

 damp regions and in years when there is much rain. The 

 loss in the United States in a single year from wheat rust alone 

 has been estimated at $67,000,000. 



98. Prevention of Wheat Rust. Although this disease 

 has been known and studied for years, it is still most difficult 

 to deal with. No practicable method has been found for the 

 direct treatment of infected wheat plants. The disease can 

 be partly checked by removing all barberries from the neigh- 

 borhood of wheat fields. Since the young wheat plant is 

 infected by spores from the barberry, and since neither winter 



