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 Epichloé 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 
