DEPARTMENT OF BOTANICAL RESEARCH. 81 



amount that it had lost the previous day. When this same plant 

 was given abundant water the stem began to rise within an hour and 

 within two days had reached the position occupied three months 

 earlier, but the relative transpiration was greater for the night than 

 for the day. Dr. Livingston had previously found that the relative 

 transpiration rates for several species of cacti is greater at night 

 than in the day, but he recorded 24 per cent of his plants as behaving 

 ''abnormally" and attributed this to pathological conditions. This 

 work seems to show that this 24 per cent was probably made up of 

 plants which were behaving normally but were under a different 

 complex of water relations from the others. The gain in weight at 

 night was large enough to attract attention and experiments are 

 planned to prove whether or not the gain is actually due to the absorp- 

 tion of water-vapor. Not until this has been completed can any 

 connection between transpiration and the movements be established. 

 The Transpiration of a Desert Tree, hy Edith B. Shreve. 



The results of the investigation of transpiration in Parkinsonia 

 7nicrophylla have been made ready for publication and are now in 

 press. Some of the main conclusions reached are as follows: 



Transpiration-rates from trees and potted plants differ so greatly 

 in amount and in hourly changes that conclusions regarding the one 

 can not be drawn from measurements of the other. 



Potted plants when grown under several conditions of air humidity 

 showed differences in anatomical structure which ranged from the 

 purely mesophytic type to the extreme xerophytic type, and trans- 

 piration measurements of these different forms showed that the 

 maximum relative-transpiration amounts varied with the degree of 

 xerophytism of the structure. Further, the maximum rate varied 

 greatly in any given plant according to its soil-moisture content. 



Both potted plants and adult trees exhibit in their daily curve of 

 relative and of actual transpiration a drop which precedes the maxi- 

 mum evaporation-rate for the day by 1 to 4 hours. This drop is 

 followed in the majority of cases in potted plants, and always in 

 trees, by a recovery. Curves constructed from measurements of 

 stomatal openings, water-content of leaves, and leaf-temperatures 

 paralleled the transpiration curve. 



From the evidences of water-content and leaf-temperature deter- 

 minations the theory is advanced that the drop in transpiration 

 occurs because the tissues have begun to dry out and the water is not 

 there to be given off at the same rate. This is really further evidence 

 of Livingston's theory of ''incipient" wilting and Renner's "satura- 

 tion" deficit. In order to account for the recovery after the drop an 

 adaptation of Dixon's theory of ascent of sap in trees is made by 

 holding that the deficiency of water in the leaves at the time of 

 increasing evaporating power of the air causes the tensile pull on the 



