50 TRANSPIRATION IN A DESERT PERENNIAL. 



dying of twigs, branches, and finally limbs, which takes place during 

 times of drought. This would happen when the tensile "pull," due to a 

 deficit of water in the upper tissues, became great enough to cause a break- 

 age of all or nearly all of the water columns. The twig would then quickly 

 die for want of water, even though the lower portions remained alive. 



On page 36 mention was made of the fact that there seems to be a relation 

 between the amount of the maximum of actual transpiration immediately 

 preceding the drop and the occurrence of the drop. This fact seems to 

 accord with the theory given above, the maximum amount of actual trans- 

 piration for any tissue being a fairly constant factor dependent for each 

 tissue upon its structure and water content, on the one hand, and upon the 

 rate at which it receives water from lower tissues on the other. 



DAILY COURSE OF LEAF TEMPERATURE. 

 METHOD.* 



The method to be described is one commonly used by physicists in the 

 determination of specific heat, heat of evaporation, etc., the underlying 

 principle being that when two substances at different temperatures are 

 placed in contact and external loss or gain of heat is eliminated, the amount 

 of heat gained by the cooler body is equal to the amount lost by the 

 warmer one. Or, expressed more concretely: 



(A) (Sa) (Ta-Tc) = B(Sb) (Tc- Tb)+C(Tc-Tb) (1) 



where 



A = weight of substance at the higher temperature. 

 B = weight of substance at the lower temperature. 

 Sa = specific heat of substance A . 

 Sb = specific heat of substance B. 

 Ta = initial temperature (centigrade) of A. 

 Tb = initial temperature (centigrade) of B. 

 Tc = final temperature (centigrade) of the mixture. 

 C = number of calories required to raise calorimeter 1 C. 



In the problem under consideration Ta, the unknown required, can be calcu- 

 lated since all the other factors can be found. 



The finding of a substance with which to bring the leaf in contact offered 

 some difficulties. An ideal substance would have to answer the following 

 conditions: (1) a liquid in which the leaf may be completely immersed, 

 which will come into intimate contact with the surfaces of the leaf, and will 

 penetrate to the interior; (2) a material of low specific heat and low density, 

 so that small differences of temperature in leaves may register large changes 

 in a small amount of liquid; (3) a non- volatile material, in order to avoid 



*A preliminary announcement of this method was made in the Johns Hopkins Univ. 

 Cir., Feb. 1912. 



