DEPARTMENT OF BOTANICAL RESEARCH. 65 



plant, and also degree of openness of the stomata. Likewise there 

 appeared an interrelation between the water-intake by the roots and 

 the water-holding capacity of the tissues and also the water-content 

 of the plant. The following conclusions may now be drawn : 



(1) The transpiring power is greatly influenced by hght intensity, 

 air-temperature, water-content of tissues, and available soil moisture; 

 these factors clearly exert their influence indirectly through their 

 action upon some internal process. 



(2) Variations in the rate of water-intake by the roots exist and are 

 evidently independent of variations in transpiration. Variations in 

 water-intake at the roots are due, on the one hand, to variations 

 in soil-retentivity, and on the other to variations witliin the plant 

 itself. The latter may be further subdivided into variations in abso- 

 lute transpiration-rate and in water-absorbing power of the tissues. 

 The variations in soil retentivity may be reduced to zero, for experi- 

 mental purposes, by the use of water-cultures or supersaturated soil, 

 and then the absolute water-intake divided by the absolute transpira- 

 tion for the same period gives quantities whose variations may be 

 traced neither to soil retentivity nor to transpiration changes. This 

 quantity, A/T, is given the name "secondary absorbing power." It 

 was found to vary in a direction which is always opposite to the varia- 

 tions in T/E for the same period, that is, T/E is greater by night and 

 A/T by day. 



(3) The water-holding capacity of cylinders cut from internal tissue 

 is less at night than during the day. It parallels the behavior of A/T 

 under all the several environmental conditions which were used in 

 the experiments. 



(4) Stomata are, in general, shut during the day and open at night. 

 Some evidence appeared that a decrease in T/E preceded the closing 

 of the guard-cells. 



(5) The above conclusions suggest the theory that the variations 

 in transpiring power and in secondary absorbing power are due to 

 variations in the water-holding capacity of internal tissue. In the 

 case of the transpiring power, the changes in water-holding capacity 

 act indirectly by causing the closure of the stomata and directly by 

 resisting the evaporative power of the air. 



(6) The source of the energy for this resistance to the evaporative 

 power of the air may be traced to the imbibitional forces of colloidal 

 gels and the cellulose walls, and hence to surface-tension forces. 



(7) The effects exerted by light intensity and air temperature, 

 especially when their duration is considered, indicate that the varia- 

 tions in water-holding capacity of tissue are due, at least in part, to 

 chemical changes brought about by the metabolic processes. Under 

 "typical" conditions, a high water-holding capacity is accompanied 

 by a low acidity, and vice versa. However, certain exceptions, occurring 



