374 



The Royal Society, London. 



Effects of moisture. 



The immediate effect of the damp atmosphere is to check transpi- 

 ration. This, in its turn, by blocking the tissues with water, disturb» 

 the normal course of metabolism, and so leads (when the light and heat 

 are sufficient) to changes in the metabolic activity of the plant, as iß 

 shown by the following facts : 



1. The outgrowths only develop if transpiration is reduced. 



2. The outgrowths are chiefly formed on organs which are actively 

 assimilating, e. g., under ordinary, red or yellow glass ; but only 

 if transpiratory actiyity is lowered : they are not formed in the 

 open. 



