MOULD FUNGI AS EXCITING AGENTS OF DISEASE. 631 



a species of nectria, which occasions the so-called cancer 

 of the pine hark, the necessary condition for the pene- 

 tration of the germinating tube is only furnished where 

 the bark has been eroded by moths ; the spores of 

 trametes pini sprout only on freshly broken surfaces of 

 branches, and from that point send their mycelial threads 

 into the wood. Frequently also it is only certain parts 

 of the plants which are suitable for the attack and 

 development of the fungi. Thus claviceps attacks 

 the flowers, exoascus the fruit, and byssothecium the 

 roots of the plants which are infected by these fungi. 

 De Bary gives an interesting example in the parasite 

 of the white rust of cresses, cystopus candidus, the 

 germinating tube of which can only develop in the 

 cotyledons, and can hence only infect those plants 

 which have just formed cotyledons at the time at 

 which the spores are being distributed and are ger- 

 minating. 



While these peculiarities in the spread of the parasitic Seasonal and 

 diseases of plants may be satisfactorily explained by 



differences in individual predisposition, we often observe distribution 



. i f i 1-1 , -i f i f epidemics 



another series of phenomena which cannot be referred O f plants. 

 to the same cause. For instance, we often see in par- 

 ticular localities an epidemic spread of a parasitic 

 disease, while in other places plants of the same species 

 are only attacked in a sporadic manner; and in like 

 manner there are years in which the disease only 

 spreads to a slight degree, while at other times severe 

 epidemics occur in the same places. 



These differences have their explanation in part in 

 certain external influences which vary with locality and 

 season, and which form the so-called local and seasonal 

 predisposition. A factor belonging to this class which 

 favours most of the infections caused by mould fungi is, 

 for example, a continuous and marked degree of moisture 

 in the air and in the soil, which of itself permits the 

 sprouting of the fungus spores. The ustilagineae, 

 claviceps, and many others, require such a continuous 

 moisture ; peziza willk., which occasions the cancer of 

 larches, only forms fruit-bearing hyphae when the air is 



