14 PLANT DISEASES 



they produce some form of reproductive body which 

 inoculates the young host-plant. In many instances 

 sclerotia are formed in living parts of the host, especially 

 in bulbs, where they remain latent until the bulb pro- 

 duces a new growth, which is infected in due course. 



From the above account it will be seen that sclerotia 

 perform the same function as winter-spores, remaining 

 passive during the resting condition of the host, and pro- 

 ducing fruit when the latter commences a new growth. 



How PARASITIC FUNGI INFECT THEIR VICTIMS. The 

 conidia or spores of numerous parasitic fungi, carried by 

 wind, rain, or other agents, are deposited on the leaves, 

 fruit, or other parts of the host-plant, germinate at once, 

 and enter the tissues. This may be considered as the most 

 direct and general method of infection. 



Armillaria mellea, an exceedingly common gill-bearing 

 fungus, or ' toadstool,' very destructive to forest and 

 orchard trees, illustrates a second mode of attack. 



The fungus often lives as a saprophyte, growing in 

 dense clusters on decaying stumps, etc. Its mycelium 

 forms thin, cordlike strands which radiate in all directions 

 in the soil ; and if one of these strands of mycelium comes 

 in contact with the living rootlets of a tree, its tissues are 

 attacked, the mycelium of the fungus enters the rootlet, 

 adopts a parasitic mode of life, spreading further and 

 further into the tissues, until finally the whole root-system 

 of the living tree is enveloped in a white, felty mycelium, 

 which gradually extends up the trunk, ending in the death 

 of the tree. 



In the meantime, numerous strands of mycelium origin- 

 ating from the diseased tree are traversing the soil in 

 every direction in search of other victims, and by this 



