214 THE BIOLOGY OF FLOWERING PLANTS 



transpiration and assimilation, summing up the reaction of 

 the plant to its environment in these two functions. This 

 relation has not yet been exhaustively studied, but some 

 suggestive w^ork has appeared. 



We may refer to Livingston's paper (1905) on the 

 " Relation of Transpiration to Growth in Wheat." He 

 found that under different conditions the increase in leaf 

 area, and so, practically, in the size of the plant, was more 

 or less proportional to the total amount of water which had 

 been transpired ; other American authors have found a 

 similar proportionality. This does not mean that transpira- 

 tion has any direct effect on growth ; it indicates rather that 

 while transpiration is active, so is the diffusion of carbon 

 dioxide, and that the amount of transpiration, as of assimi- 

 lation, is a function of leaf area. No more striking illus- 

 tration of another relation of transpiration to assimilation 

 could be found than the result of Thoday (19 10) we have 

 already quoted. The assimilation of the sunflower leaf 

 steadily decreases as transpiration reduces the turgor of 

 the leaf, and ceases when the leaf is completely wilted. 



We may refer to an investigation by Iljin (1916) on the 

 relation of assimilation to transpiration in the plants of 

 Russian steppes and neighbouring meadows and ravines. 

 Assimilation was determined in an enclosed atmosphere 

 rich in carbon dioxide, and transpiration from cut shoots 

 dipping in water ; both are related to unit dry weight. 

 The methods are open to objection, and the plants were not 

 in natural conditions, yet certain general features are 

 brought out. In its natural environment a mesophyte such 

 as Geranium pratense or Senecio doria transpires at about 

 the same rate, or rather less rapidly, than does a xerophyte 

 such as Phlomis pungens or Stipa capillata in its natural 

 habitat. Transferred to the steppe environment the tran- 

 spiration of the mesophyte far exceeds that of the xerophyte. 

 This is illustrated by Table XXXII. 



