FUNGI. 203 



extensively cultivated, and less associated with the 

 supplies of human food. No one who has had the 

 experience of any of these pests amongst his lettuces, 

 onions, tomatoes, or in his clover field would estimate 

 lightly their powers of destruction. 



Another group which may be referred to, in passing, 

 are represented by the hop-mildew and the English 

 vine-mildew. In this family of the Erysiphei the 

 mycelium is developed on the surfaces of the leaves, 

 and the host plants are injured by a kind of suffoca- 

 tion, through the obstruction of all the air-passages. 

 The results are no less disastrous and fatal than in 

 those cases where the mycelium penetrates the tissues. 

 About 120 species are known, and all are destructive. 



The common fly-mould may be taken as the type 

 of a considerable number of parasitic fungi which 

 establish themselves upon the bodies of living insects, 

 and replace all the tissues by mycelium, which soon 

 causes death. Many of these appear as an epidemic 

 to which hosts of flies and other soft-bodied insects 

 fall a ready prey. Although vastly different in their 

 structure and habits, the aquatic moulds, such as 

 Saprolegnia, which is the moving cause of the salmon 

 disease, may be mentioned as carrying on the work 

 of destruction. 



Amongst imperfect fungi, such as the moulds 

 {^HypJiomycetecs), the majority are probably sapro- 

 phytic, and only assist materially in the disintegra- 

 tion of dead substances ; but some entire genera are 

 parasitic, and attack living plants. Of these latter 

 we might enumerate Ramiilaria and Cercospora as the 

 most extensive and most fatal. It is unnecessary to 



