66 PHYSIOLOGICAL RELATIONS 



promoting a healthy growth of all parts of the host throughout the 

 season would decrease disease. 



After all, saprophytism and parasitism are terms of degree, and 

 organisms are classed as possessing the one habit or the other 

 simply upon general evidence, or macroscopic appearances. It is 

 quite possible, however, that in an ultimate analysis of the associa- 

 tion of host and parasite, or of the method of fungous attack, 

 many organisms now regarded as parasitic would be found to show 

 a true saprophytic habit. It is possible that such organisms may 

 gain entrance to the host through injuries ; in other words, estab- 

 lish themselves in association with dead cells. By growth in these 

 cells the excretion of acids and other diffusible products might 

 bring about death in other cells in the vicinity, to which the fungus 

 eventually spreads. So far as the actual presence of the fungus is 

 concerned, therefore, there would be no direct association with a 

 living cell in order to secure organic nutrients. This method of 

 attack has been demonstrated to be that followed by Botrytis cinerea, 

 a common greenhouse fungus. 



II. GENERAL RELATIONS TO CLIMATOLOGICAL FACTORS 



Experiment and observation alike demonstrate that the abun- 

 dance of a very large number of fungous diseases is directly con- 

 nected with or conditioned by climatological factors. With respect 

 to conditions in the open, climatological factors are generally under- 

 stood to mean water (moisture), light, temperature, and wind. 

 These factors may affect independently host and parasite, and they 

 may affect the interrelations of these organisms. Moreover, it is 

 often difficult to interpret what factor is finally operative, or it is 

 difficult to determine what are direct and what indirect effects of 

 these environmental conditions upon host and fungus. In many 

 instances it would be merely hypothetical with the data at hand — 

 largely observational — to do more than designate certain apparent 

 or proximate causes. 



Moisture. Many fungous diseases are directly associated with 

 abundant precipitation, or a humid atmosphere. There is no more 

 conspicuous example of this than the brown rot of stone fruits — 

 a disease which, in moist weather, has repeatedly crippled the 

 peach industry. The association of epidemics of such diseases as 



