778 JOURNAL OF FORESTRY 



controlling transpiration, and that evaporation coincides with transpira- 

 tion more closely than any of the remaining factors. The reason is, of 

 course, that evaporation is itself dependent on the forces affecting 

 transpiration (solar radiation, temperature, relative humidity, and 

 w^ind), though not in exactly the same way. Transpiration virtually 

 ceases at sunset, while evaporation diminishes more gradually. 



A low rate of evaporation causes greater economy in the use of water 

 by the plant — that is, in a humid climate the plant uses less water per 

 gram of dry weight and of ash produced than in a dry climate. Sim- 

 ilarly a moist season will result in considerably greater economy of 

 water than a dry season. Briggs and Shantz had already shown this 

 in their water requirement studies at Akron. 



The effect of soil moisture on transpiration (charts on pp. 103 and 

 104) is one of the most interesting parts of the investigation. It was 

 found that transpiration was greatest at the optimum water content. 

 Too much water, contrary to expectations, decreases transpiration, 

 sometimes even below that shown by the low water content. Kiessel- 

 bach also shows (p. 140) that with a low soil moisture the plant uses 

 a little less water per gram of dry matter, of grain, and of ash produced, 

 but that the total yield with the low soil moisture is about a third less 

 than with the optimum moisture content. With an excess of water 

 the yield was reduced almost as much as with a deficiency of water, 

 and the plant used more per gram of dry matter produced than with 

 optimum water conditions. 



Soil fertility has a distinct effect upon the water requirement, a good 

 soil causing the plant to use more water by making it more vigorous, 

 but causing it to yield more ; so that actually on the good soil the plant 

 uses less water per unit of dry weight produced than it uses on a poor 

 soil. Thus the poor soil allows less water to be drawn out ; but, since 

 it permits the plant to make less with the water used, is more wasteful 

 of water than a fertile soil. This is a matter of considerable impor- 

 tance in explaining some of the forces underlying competition on dif- 

 ferent soils. Certain soils are fertile to one tree and infertile to an- 

 other ; the tree to which the soil is infertile will use water less econom- 

 ically than the other tree, and will therefore be at a disadvantage for 

 water as well as for nutrients. 



Of considerable interest in reforestation is the experiment on the 

 influence of a change in the relative humidity upon transpiration. 

 Kiesselbach grew two sets of plants in greenhouses for eight weeks, 

 one set in dry air, the other set in moist air. Those in the dry air 

 showed a markedly higher rate of transpiration than those in the moist 



