TRANSPIRATION. 149 



XII. 



TRANSPIRATION. 1 



TRANSPIRATION means the evaporation of water 

 from a plant. Let us imagine a plant which con- 

 tains as much water as it is capable of holding, 

 and is in contact with an available supply of 

 water in the ground. Each cell contains its full 

 amount of liquid, and, if no water is lost, no water 

 will be absorbed by the plant. But now suppose 

 that evaporation begins to take place from the 

 cells which are in contact with the atmosphere. 

 As the cell-walls allow liquids to pass freely, these 

 cells immediately begin to draw on the water sup- 

 ply of their neighbors, and these, in turn, on the 

 cells adjoining them, and so on, till throughout 

 the whole plant there is an ascent of water to 

 supply the place of what has been lost. Finally 

 the call is made on the rootlets, and these promptly 

 answer the demand by sucking in the needed liquid 



1 In compiling this article the editor has availed herself largely of 

 the charming chapters on transpiration in " Pflanzenleben." The book 

 contains many interesting suggestions on this subject, besides those 

 which are given here. 



