78 DISEASES OF FIELD & GARDEN CROPS. [OH. 



CC, represent the lower cuticle of the leaf, whilst the 

 openings at D D D, show the stomata or organs of trans- 

 piration. It will be observed that the spawn of the 

 invading fungus does not enter the stomata or traverse the 

 intercellular spaces of the leaf, such as are shown at 

 E E E. The barrel -shaped spores or conidia of this 

 fungus are so numerous that more than 10,000 are 

 produced on every square inch of leaf surface, and every 



FIG. 28. 



Oidium Balsamii, Mont. 

 Germinating Spore enlarged 1000 diameters. 



turnip leaf will carry on its two surfaces a million or 

 more of these reproductive bodies. 



The spores germinate very readily, for they have only 

 to be dusted on to clean glass and kept moist, under a 

 bell-glass, when they will be seen to germinate at once as 

 illustrated at Fig. 28, enlarged 1000 diameters. The 

 germinating conidium bursts at one corner as at A, and 

 from this corner the contained protoplasm or vital 



