632 .VITAL ACTIONS OF THE LOWER FUNGI. 



Factors which moist. The duration of the period of moisture is often 



influence the ., . . ., . -, ,,. . 



spread. the important point ; thus epidemics 01 rosemma quer- 



cina develop only when there is continuous rain ; the fruit 

 carriers of hypoderma macrosp. swell up and burst only 

 when rain has lasted for several days. At times also 

 the temperature must be specially favourable for the 

 sprouting of the spores, and they can only cause infec- 

 tion when suitable temperature and suitable moisture 

 happen to be present together. The conditions for a 

 successful infection are still further complicated, in that 

 the above-mentioned factors which predispose to the 

 penetration of the fungus mycelium into the plant must 

 coincide with a certain stage of development of the 

 fungus, and with external conditions which are favour- 

 able to this development. A moist warm season is 

 often a predisposing factor which favours the growth of 

 the parasite when young plants are present, or when, at 

 the same time, insects have caused injuries of the 

 epidermis, or when, as the result of storm, &c., fresh 

 fractures of the branches have been produced. If these 

 predisposing factors are absent during the moist period, 

 or if the latter occurs when the plants in question are 

 older and provided with firmer epidermis, no infection 

 occurs. Here and there other more remote factors, 

 which often seem to be a matter of chance, play a part 

 in the development, and more especially in the epidemic 

 spread of the parasitic diseases of plants. Thus the 

 development of the uredineae depends upon the condition 

 of their second host, which they require as the result of 

 their alternation of generation, and if the barberry 

 bushes are accidentally or intentionally removed the 

 rust of grain ceases ; in many species of fungi suitable 

 means of transport must aid the spread of the spores, 

 as in the case of phytophtora fagi, where men and 

 animals in passing transport the spores. Lastly, in 

 some cases where infection can only occur in the 

 immediate neighbourhood from the further spread of 

 the mycelium, everything depends on the grouping of 

 the plants which can be attacked ; it is only when these 

 occur in close contact with each other that an epidemic 



