NATURE OF DISEASE. 135 



to make the hyphae on one side of an artificial 

 membrane e.g. collodion penetrate it by placing 

 one of these attractive {chemotropic) substances 

 in suitable proportions on the other side. The 

 hyphae dissolved holes in the membrane by means 

 of enzymes and plunged into the attractive sub- 

 stance on the other side. 



The foregoing sketch gives us a glimpse into 

 the causes at work in parasitism. 



Suppose a fungus on the outside of the epi- 

 dermis of a young organ say a leaf It may be 

 unable to penetrate into the plant, and finding no 

 suitable food outside it dies : or it may be satisfied 

 with the traces of organic matter on the epidermis 

 and then lives the life of a saprophyte. Or 

 it may be able to establish a hold-fast on the 

 tender epidermal surface, but without entering 

 the cells, and irritate the developing organ by 

 contact stimulation, inducing slight abnormalities ; 

 if in its further, purely superficial growth such 

 an epiphyte covers large areas of the leaf, and 

 especially if the hyphae are dark coloured e.g. 

 Dematium and other " Sooty Moulds " injury 

 may be done to the leaf owing to the shading 

 action which deprives the chlorophyll below of 

 its full supply of solar energy. Some epiphytes, 

 however, are able to fix their hyphae to the 

 epidermis by sending minute peg-like projections 

 into the cuticle Trichosphaeria^ Hevpotrichia 

 while others send haustoria right through the 

 outer epidermal walls e.g. Erysiphe and thus 

 supplement mere contact-irritation and shading by 



