SOIL MOISTURE AND TO EVAPORATION. 



65 



this table that the efficiency of the regulative response varies from one- 

 sixth to one-twelfth, and this without apparent relation to nyctitropic 

 movement; for one plant of Tribulus, with marked nyctitropic move- 

 ment, shows an efficiency of one-twelfth and the other an efficiency of 

 one-eighth, and Boerhavia, without appreciable nyctitropic movement, 

 shows a variation in efficiency of from one-sixth to one-twelfth. 

 Further work will need to be done in this field of inquiry before any 

 definite conclusion can be reached. 



TABLE XIV. Summary of Transpiration Experiments. 



*These efficiencies are obtained from the maxima and minima ratios derived from the rate 

 of transpiration from the entire plant and that of evaporation from the whole evaporimeter 

 surface. See the discussions of the experiments. 



A table of the rates of water loss from these plants will be given in 

 the following section, together with data concerning the moisture con- 

 tent of the soil at the end of the experiment. 



WATER REQUIREMENT OF CERTAIN DESERT PLANTS. 



In the present section will be presented what data were obtained 

 bearing upon the amount of water needed in the soil in order that 

 plants may live in the desert. This problem was attacked directly, by 

 determining the water content of the soil samples in which the plants 

 for the foregoing transpiration measurements had been growing. This 

 was the sole end in view when the first of these experiments were 

 started, the intention being merely to relate the moisture content of the 

 soil to the rate of transpiration both for the entire plant and for unit leaf 

 surface. The data on regulation of water loss, presented in the last 

 section, developed as a secondary consideration in the course of the work. 



The results of the moisture determinations of the soils are pre- 

 sented in Table XV. In this table the first two columns again give 

 the numbers and subjects of the experiments. In the two following 

 columns are given average hourly rates of transpiration for entire plant 

 and for one square centimeter of leaf surface, these being calculated 



