FUNGAL PARASITES 179 



injuriously affect the development of the 

 cuticle of the leaf. Or, again, it may lead to 

 an excessive amount of watery sap in the 

 superficial tissues, quite apart from the effects 

 of external moisture on the outer surfaces of 

 the stems and leaves. It is well known that 

 bad cultural conditions may predispose plants 

 to disease, and observation teaches that some- 

 times, at any rate, the effects are due to 

 imperfect development of the tegumentary 

 tissues. 



The presence of nitrogenous manure in 

 excessive quantities, in proportion, that is, 

 to the other nutritive constituents of the soil, 

 is another predisposing cause of fungal attack. 

 It operates in several ways, but often in- 

 directly by causing an undue accumulation 

 of soluble nutritious substances in tissues 

 and cells the walls of which are imperfectly 

 thickened. 



Starvation of an essential food constituent 

 may act as a specific cause of predisposition. 

 Thus many grasses, when they are grown on 

 land in which the supply of potash salts is 

 inadequate, become very liable to epidemic 

 attacks of a fungus known as Epichloe typhina. 

 The disease makes its appearance in the form 

 of white (changing to yellow) zones situated 

 just above the knots of the stem, and extend- 

 ing upwards for a centimeter or two. These 

 zones mark the regions where the reproductive 

 organs of the fungus are formed. A poor 

 supply of potash is also known to affeet the 



