164 



THE BOOK OF WHEAT 



rain or dew furnishes sufficient moisture for germination. A 

 small thread or filament is then sent out, which requires but 

 an hour or two to pass through a breathing pore of the wheat 

 plant, or, in the absence of a convenient breathing pore, to 

 bore its way into the stem or leaf, within which a mycelium 

 is formed. An ordinary dewdrop may contain hundreds of 

 secidiospores that have been wafted to it upon the air. The 

 time required for the rust to break out as a spot or pustule, 

 after the germination of the infecting spore, varies from 8 to 

 14 days, for it is dependent upon atmospheric conditions. This 

 breaking out through the skin of the wheat 

 plant is the result of the great numbers of 

 ovoid spores that are formed, and these red 

 summer spores, the uredospores, are thus 

 enabled to drop off and float away upon the 

 air to other wheat plants. If moisture is 

 present they germinate at once, and the en- 

 tire above process is repeated. Several gen- 

 erations of the red spores may be formed 

 during one growing season. Countless myriads 

 of spores are thus produced, a pustule 1-16 

 by 1-64 of an inch in size containing over 

 3,000 such spores. Under favorable conditions 

 the rustiness of the grain increases with mar- 

 velous rapidity. In the meantime the spore 

 beds which produced the first red spores are 

 not inactive, but are producing teleutospores, 

 that is, the black winter or resting spores. 

 These are thick-coated, Indian-club shaped, 

 and two-celled. They may now also appear 

 in new spore beds in which no red spores 

 have first formed. Over 2,000 of these spores 

 have been counted in a pustule 1-16 by 

 1-48 of an inch in size. No rest- 

 ing spores have been observed to germinate until late in the 

 following winter. When they sprout a germ tube (pro-mycelium) 

 proceeds from each of their two cells. These germ tubes soon 

 divide into four cells which immediately produce severel mi- 

 nute, delicate cells known as sporidia. If sufficient moisture 

 is present, the sporidia will germinate at once. If not, many 



BLACK AND RED 

 RUST 



