WATER UTILIZATION BY TREES 51 



to 20 percent. Immediately the transpiration rate increased to the 

 normal before the water supply was cut off, and a month later new 

 shoots appeared. This is a remarkable example of the way in which 

 plants are able to check their water loss through transpiration to 

 adapt themselves to a decreased water supply, but, unfortunately, not 

 all plants are able to adapt themselves so efficiently to an unfavorable 

 soil-moisture content. 



Dole (52) found with 3-year-old potted plants of Pinus contorta that 

 transpiration, when the soil-moisture content is 20 percent, is two to 

 seven times as much as when the soil-moisture content is only 10 per- 

 cent, in the case of sandy loam. Busgen and Munch (33, p. 304) 

 report that ash used 56,689 kg of water per 100 g of leaf substance 

 when given just enough water to remain alive and 100,077 kg when 

 watered abundantly. Corresponding figures for pine were 5,802 and 

 12,105 kg. 



The studies on soil-moisture content as related to transpiration 

 have been summed up by Baker (6, p. 48) and indicate that: 



1. In the presence of plentiful soil moisture — 



(a) transpiration falls when stomata close on account of the darkness of 



night; 

 (6) during daylight hours transpiration is closely correlated with the 



same factors as influence evaporation. 



2. With deficiency of soil moisture — 



(a) transpiration falls on account of stomatal closure; 



(b) transpiration also falls even if stomata remain open, due to some foriii 



of mechanical or physiological control. 



COMPOSITION OF SOIL SOLUTION 



Not only is the quantity of soil water important, but the composi- 

 tion also may affect transpiration. Of the various common soil-water 

 components only two, humins and salts, will be mentioned here, these 

 because of their silvicultural importance. In most forest soils there is 

 an extremely high percentage of organic matter in various stages of 

 decomposition. To test the effect of these humous compounds on 

 transpiration, 50 g of humus was mixed with 500 g of distilled water 

 and filtered. In the filtrate, which contained 0.041 percent of organic 

 substance and 0.018 percent of inorganic materials, were placed some 

 herbaceous plants, as well as branches of Taxus. All showed a decrease 

 in transpiration of 10 to 30 percent compared with plants in distilled 

 water, as reported by Burgerstein (28, p. 147)- 



The effect of composition of soil water on the transpiration of trees 

 has been very little studied. There have been, however, many papers 

 on the effect of salt on the transpiration of herbaceous plants, most of 

 which indicate that an increase in salt content decreases the transpira- 

 tion. Various reasons have been assigned for this, all of which may 

 play a role, but doubtless the most important factor is the change in 

 osmotic pressure. Materials which increase the osmotic concentra- 

 tion of the cell sap decrease the ease with which water can be with- 

 drawn, and consequently they decrease the transpiration. 



Minor Factors 

 air pressure 



Among the minor factors that have been studied in connection with 

 the transpiration of trees is air pressure. Variations in air pressure at 

 any one spot are so slight as to be negligible in transpiration studies; 

 but if one wishes to compare transpiration values at different altitudes, 



