176 DRY-FARMING 



a free water surface is not the same as that from 

 plant leaves may be shown in a general way from the 

 fact that the amount of water transpired from a 

 given area of leaf surface may be very much larger 

 or very much smaller than that evaporated from an 

 equal surface of free water exposed to the same con- 

 ditions. It is further shown by the fact that whereas 

 e"\'aiooration from a free water surface goes on with 

 little or no interruption throughout the twenty-four 

 hours of the day, trans]nrati(3n is virtuall}' at a stand- 

 still at night even though the conditions for the rapid 

 evaporation from a free water surface are present. 



Some of the conditions influencing the transpira- 

 tion may he enumerated as follows : — 



First, transpiration is influenced by the relative 

 humidity. In dry air, under otherwise similar con- 

 ditions, i^lants transj)ire more water than in moist air, 

 though it is to be noted that even when the atmos- 

 phere is fully saturated, so that no water evaporates 

 from a free water surface, the transpiration of plants 

 still continues in a small degree. This is explained 

 by the observation that since the life process of a 

 plant ])roduces a certain amount of heat, the plant 

 is always warmer than the surrounding air and that 

 transpiration into an atmosphere fully charged with 

 water va])or is consequently made possible. The 

 fact that trans]:)iration is greater under a low relative 

 humithty is of greatest importance to the dry-farmer, 

 who has to contend with the dry atmosphere. 



