Iv] Mildews 53 
remain throughout the winter in their cases on the 
dead leaves or in the soil. 
They are protected from injury due to climatic 
conditions by the walls of the spore case. In spring 
the. cases burst and the asci are liberated. The walls 
of these also split and the ascospores are set free, and 
under suitable conditions germinate, sending out a 
short tube capable of infecting growing host plants. 
They germinate most readily at a temperature of about 
50°F. The mycelium which they produce buds off 
conidia during the spring and summer. The damage 
done by this mildew to cereals in a wet season may 
be considerable, arising not only from the food which 
the plant gives to the fungus, but also from the 
limiting of the work done by the leaves in manu- 
facturing food from the air. The normal amount of 
light is prevented from falling on the leaf by the 
presence of the mycelium. The amount of air which 
ordinarily enters the pores of the leaf is also reduced. 
The diseased leaves become yellow, and die before 
the normal ones, and hence their functions cease 
earlier, with the result that the total yield is lower. 
The fungus known as Erysiphe graminis com- 
prises a number of different varieties; and each one 
may be regarded as being distinct from the others, 
although no difference has yet been found in their 
structural appearance. The different varieties are 
distinguished by the plants which they attack. One 
variety attacks only wheat, another barley and 
sometimes wheat to a very slight extent, another 
attacks oats and a related grass. A cereal following 
another cereal crop which has been badly attacked by 
mildew will suffer no more than if it had followed a 
