152 DISEASES OF FIELD & GARDEN CROPS. [OH. 



germ tube of the rust fungus enters the tissues of the 

 host plant through these apertures, just in the same way 

 as it would enter a small hole in a microscopic slide or go 

 over the edge of a slip of glass, as we have many times 

 seen it do. A stomate belonging to wheat is very large 

 in comparison with the size of a spore \ the length of an 

 organ of transpiration is from G to H. 



When once the Uredo mycelium has found its way 

 amongst the tissues of the wheat plant, the germ tube is in 

 its natural position ; it now branches right and left, and 

 ramifies amongst the green constituent cells of the leaf. The 

 mycelium here quickly produces new Uredo pustules, which 

 burst through the wheat cuticle in fresh places, so that a 

 wheat leaf which may have had only a few pustules in 

 June, may have the number more than quadrupled by July 

 by the continued germination of the Uredo spores on the 

 leaf surface. No doubt the mycelium also spreads in the 

 leaf from the base of the original pustules. 



Before proceeding farther it must be noted that these 

 Uredo spores are very short lived, for if they germinate on 

 any unsuitable material they quickly perish. When all 

 the vital material is once poured out of the spore into the 

 germinal tube, growth can proceed no farther on an un- 

 suitable matrix. It is only when the spores germinate 

 on grasses, and the germ tubes find their way through the 

 organs of transpiration into the tissues of the supporting 

 plant, that the plasma of the fungus is able to continue its 

 existence. It is therefore obvious that when the frosts of 

 winter arrive and grasses are for the most part dead, or, 

 if alive, sluggish to the last degree, that this fungus, 

 which has a much more slender hold on life than any 

 grass, must perish. Unless the mycelium of the rust 

 fungus has moderate warmth, sufficient moisture, and the 

 interior of a living grass, it probably always collapses and 

 perishes. 



As in the spring rust of wheat, the life of the fungus of 

 summer rust and mildew is carried over the winter in the 



