686 FUNGI : GENERAL 



compounds upon the direction of growth, not only influences 

 the hyphae of parasitic species but the hyphae of Penicillium 

 glaucum, and other saprophytic fungi may be similarly induced 

 to grow into tissues they would not ordinarily penetrate when 

 these tissues are specially charged with excess of certain nutritive 

 substa,nces. Large amounts of water, sugar, and several other 

 compounds within the cells of the host make such a plant very 

 liable to the attack of fungi. 



Parts of plants which are young and whose external cell-walls 

 have not yet become cuticularised are often specially liable to be 

 invaded by fungi, whereas older portions of the same plant 

 possessing a well-developed cuticle or a layer of cork-cells escape 

 infection. 



The extent and character of the damage done to the host 

 plants by parasitic fungi is very variable. In many instances 

 the cells of the tissues permeated by the fungus are killed and 

 disorganised. In such cases the amount of the injury depends 

 upon the extent of development of the myceUum ; where the 

 latter is confined to the immediate neighbourhood in which the 

 fungus makes an entrance, the injury is very localised and 

 amounts to little more than a discoloured spot on the leaf or 

 other member of the affected plant ; when the mycelium spreads 

 extensively the damage to the tissues of the host is corre- 

 spondingly great. 



Instead of destroying the cells of their host, some parasites 

 have a stimulating effect upon the tissues with which they are in 

 contact, and the organ attacked, and parts near it grow more 

 rapidly and become larger than similar parts of the plant which 

 are free from the fungus. The increased size of such deformed 

 parts is most generally due to energetic cell-multiplication 

 induced in them. 



Abnormally enlarged tissues are said to be hypertrophkd, the 

 whole malformation, whatever form it assumes, being known as 

 a fuHgus-gall. 



lo. General advice to be followed when dealing witli plant- 



