88 NITROGENOUS MANURES [chap. 



its appearance it is very difficult to keep in check and 

 often destroys the whole crop with great rapidity ; as, 

 for example, has been the case with the leaf spot fungus 

 Cercosporium inelonis, which has of late years proved so 

 destructive to cucumbers grown under glass. 



Various attempts have been made to get a little 

 nearer to the cause of this association of high nitro- 

 genous manuring with susceptiblity to disease. In the 

 first place, certain physical differences can be traced 

 in the tissues of the plants; just as high nitrogen 

 results in a weakness of straw in cereals, due to a 

 long-jointed soft stem, so the cuticle of the leaf and 

 the cell walls of the leaf tissue are measurably thinner 

 when the plant has been grown with an excess of 

 nitrogen. The cause is, however, more probably to be 

 found in some alteration in the composition of the cell 

 sap, which renders it a better medium for the growth 

 of the fungus in question. It has been found, for 

 example, that spores of the Uromyces betae will grow 

 freely upon a bruised surface of the mangold leaves 

 grown with excess of nitrogen, but make no headway 

 when sown upon a similarly bruised surface of the leaf 

 of a normally manured plant 



The softness of tissue that is induced by large 

 applications of nitrogenous manure — most markedly by 

 nitrate of soda, because of its immediate availability — 

 is recognised in other ways ; for example, cabbages and 

 similar vegetables grown rapidly with nitrate of soda 

 are preferable for immediate consumption because of 

 their tenderness, but in the market they bear a bad 

 reputation, because the same softness of tissue leads 

 to rapid wilting and a faded appearance when the 

 vegetables have been cut for some time and have 

 experienced the usual amount of rough handling in 

 transit. 



