RELATIVE TRANSPIRATION 



165 



be eliminated, and there is exhibited the physiological be- 

 haviour of the leaf — that is, the difference in reaction shown 

 by the leaf from that of a physical evaporating surface. This 

 is not always true, for Knight (19 176) has shown that different 

 strengths of wind do not affect the plant and the atmometer 

 in the same way, even directly ; that is, evaporation from 

 different types of physical surface does not keep a constant 

 relation in still and moving air. If the wind velocity is 

 constant throughout an experiment, however, the relative 

 transpiration graph does reflect the physiological behaviour 

 of the plant as related to changes in such factors as tempera- 

 ture and light. The same result is secured if, instead of 



Fig. 15. — Transpiration (broken line) of Euphorbia capitellata through 

 three days, compared with evaporation (continuous line) and relative 

 transpiration (dot-dash line) : the scale of the ordinates is different in 

 the three graphs. (After Livingston, modified.) 



reducing transpiration and evaporation to unit areas, we 

 simply divide transpiration from any (constant) area by 

 evaporation from any (constant) area. The value so obtained 

 may be referred to as the T : E ratio. It is, of course, 

 proportional to relative transpiration. Only occasionally 

 has the condition of constant wind velocity been observed in 

 determinations of relative transpiration, and results must 

 therefore be interpreted with caution. 



Example o£ Relative Transpiration.— In Fig. 15 are 



