PATHOLOGY 77 



subdue. Still epidemics arise, and "rust," "smut," 

 and " scab " are still known to the farmers. Potato 

 rot and peach curl, spoiled fruit and wasted turnips are 

 due to parasitic fungi. The pathological effect on the 

 host plant varies with the kind of disease. In some 

 eases its life is drained away with almost no outward 

 sign, in others the presence of the fungus acts as an 

 irritant, and abnormal swellings or discoloured lumps 

 are produced by the stimulated tissue cells. These 

 correspond to some extent to the tumours and swellings 

 that occur in the tissues of animals. 



Such swellings are also produced by animals in the 

 plant tissue. These are often harmless enough, and 

 merely locally disfigure the leaf or branch without 

 materially affecting the whole individual. Such are 

 most " galls " which are formed by insects depositing 

 their eggs in the plant tissue, whose larvae develop there 

 and with their growth stimulate an abnormal develop- 

 ment of the plant cells. These pathological tissues are 

 often extremely interesting, and are developed with 

 characteristic regularity according to the insect stimu- 

 lating them. Zones of woody and fibrous coloured cells 

 are developed pathologically in the soft tissues of leaves 

 for instance, which would normally be incapable of 

 forming any such cells. 



A third type of attacking organism with which the 

 ordinary plant has to contend is the higher parasite of 

 which the Dodder (Cuscuta) is a well-known illustration. 

 This pest belongs to one of the highest orders of the 

 flowering plants, but by reason of its parasitism it has 

 degenerated to a mere colourless thread which sucks 

 all its nourishment from its host. It does its own 

 flowering, however, and produces seeds which shortly 

 after their germination begin their course of aggression. 



