THE SPREAD OF DISEASE 217 



the cool house, and in a comparative absence of moisture 

 in the air, the fungus was held in abeyance, and a crop 

 grown under such conditions would have matured without 

 a trace of obvious disease, although the disease known to 

 be present quickly manifested itself when the plants were 

 placed under conditions that favoured the rapid develop- 

 ment of the fungus. On the other hand, in the three plants 

 that were grown under conditions specially favourable to 

 the fungus, the disease quickly manifested itself. The 

 same relative conditions occur in nature. Every grower 

 of potatoes can predict with almost unerring certainty the 

 advent of potato blight, the sign being damp, warm, cloudy 

 weather. Under such conditions it is highly probable that 

 the sudden outbreak of disease is not the result of infection 

 by spores, but rather due to the presence of the fungus 

 already in the plant, that originated from hybernating 

 mycelium, suddenly stimulated to action by favourable 

 weather conditions. 



In many instances, more especially in the Lichens, the 

 balance between parasite and host has become so thoroughly 

 adjusted that no injury is experienced by either com- 

 ponent ; on the other hand, mutual benefit is derived from 

 the combination. When this . condition of things is 

 attained it is expressed by the term symbiosis or mutualism. 

 Symbiosis between fungi and flowering plants is not 

 common, but a striking instance is recorded by Freeman 

 as existing between certain fungi and three kinds of rye- 

 grass respectively, Lolium temulenlum, Z. perenne, and L. 

 italicum. In L. temulentum, or darnel, the life-history of 

 the fungus is briefly as follows. The mycelium of the 

 fungus is present in the ' seed ' ; on germination this 

 mycelium also commences growth, and keeps pace with 



