8 



SPRAYING CROPS 



to the roots of the higher plants. As there is Httle 

 nourishment to be obtained betzceeii the cells, the 

 m}xelium develops minute processes which push 

 through the cell walls and absorb the cell contents. A 

 small section of an affected leaf, greatly magnified, 

 is represented in the accompanying figure, the un- 

 shaded double-walled spaces representing the leaf 

 cells, the shaded part between the walls the mycelium 

 of the fungtis, and the projections marked a, a, the 

 processes or suckers that penetrate the cells. 



After this mycelium has developed in the leaf for 

 some time, it is ready to produce its spores. Conse- 



quently it sends out through the breathing pores or 

 stomata of the leaf its fruiting branches. These bear 

 upon their tips small oval bodies which are the spores. 

 Some of these fruiting branches are represented mag- 

 nified on the next page. The ''mildew'' visible to the 

 naked eye is composed of these fruiting branches and 

 their spores. It only develops under certain atmos- 

 pheric conditions : so that the mycelium may exist in 

 the affected parts of the vine for some time before this 

 outward manifestation of its presence occurs. Besides 

 the spores above described, which are produced during 

 the summer season, and conseqtiently are called sum- 

 mer spores, there is developed in autumn a dift'erent 

 class of spores, by which the fungus passes through 



Section of Leaf Showing' Mycelium of Fungus 



Magnified. (After Farlow) 



