DISCOVERY OF MARQUIS WHEAT 177 



starch. When the fungixs has been growing within the 

 tissues of the wheat-plant for about a week, red powdery 

 pustules break out, particularly upon the leaf -sheaths and 

 stems. These may become gradually larger and more con- 

 spicuous, and their number may increase as the infec- 

 tion spreads ; but, sooner or later, they pass away and are 

 replaced by black pustules which persist on the straw 

 through the winter. Both the red and the black pustules 

 consist of spores which resemble in function the seeds of 

 Flowering Plants, for they serve to reproduce the parasite 

 and permit of its continuing its life-cycle. The action of 

 the Rust-fungus upon the wheat-plant causes a greater 

 or lesser shriveling of the grains. The consequence of 

 an attack of Eust to the farmer, therefore, is a greater or 

 lesser diminution of the yield of wheat at harvest time. 

 Xow, during the great Rust epidemic ^^ of 1916, it was 

 observed in many localities that the fields of wheat which 

 ripened earliest, suffered least from the disease. This is 

 easily explicable if one realizes: that the Rust-fungus 

 is a plant, that it therefore requires food for its growth, 

 and that it naturally flourishes best in wheat-plants which 

 are richest in sugars and proteins. When the fungus at- 

 tacks a younger wheat-plant, with its grains only in the 

 milk or in the dough stage, the straw is still rich in all those 

 substances which enable the parasite to vegetate luxuri- 

 antly, for these have not yet been transferred to the heads. 

 On the other hand, when the fungus attacks an older 

 wheat-plant, with its grains ripening, the straw is well ad- 

 vanced toward exhaustion, for a considerable volume of 

 its sugars and proteins has already passed up into the 

 grains which in consequence have become partially filled. 



38 Plant pathologists in North America are now discarding the 

 term epidemic as applied to plant diseases and are using the term 

 epiphytotic instead. To avoid misvmderstanding, I have not adopted 

 this innovation here. 



