DEPARTMENT OF BOTANICAL RESEARCH. 101 



in a late publication. The euphorbias are all natives of the South 

 African desert and do not grow naturally in the American deserts. The 

 ratio A/T (water absorption by roots divided by water-loss by trans- 

 piration for the same period) was calculated from experimental data 

 and found to show in general the same behavior as for cacti, i. e., the 

 ratio is greater for the day than for the night under some conditions. 

 Accompanying this behavior there is an upward movement of branches 

 for the day and a downward one at night. However, neither the differ- 

 ence between the day and night A/T ratios nor the movement of the 

 branches was as great as in the case of the cacti. Furthermore, there 

 were more exceptions to the general behavior than appeared in the first 

 experiments with the cacti. The exceptions for Euphorbia have not 

 been dealt with. 



Seasonal Changes in the Water Relations of Desert Plants, by Edith B. Shreve. 



For a number of years experiments have been conducted on a success- 

 ful desert perennial, Encelia farinosa, and on two equally successful 

 desert annuals, Streptanthus arizonicus, a winter annual, and Amaran- 

 thus palmeri, a summer annual, with a view to discovering any changes 

 that may occur in their water relations with the march of the seasons. 

 The work has now reached a stage where the following facts have 

 appeared : 



In the case of Encelia, the total transpiration per unit area for 24 

 hours was only 1.4 times as great for May as for January and Septem- 

 ber, while the total evaporation for 24 hours, as measured by the white 

 atmometer, was twice as great for May as for January and 1.8 times 

 as great for September as for January. In May the soil-moisture was 

 only 0.46 of its value in January and by September it had risen to 1.1 

 of the January amount. If the severity of environmental conditions 

 with reference to water be represented by the ratio of evaporation to 

 soil moisture (E/Sm), then the conditions were nearly 4.5 times as 

 great for May as for January and only 1.6 times as great for September 

 as for January. The maximum leaf water-content in January was 3.5 

 times greater than in May. The average daily variation in January 

 was from 5.3 to 3.6 grams of water per gram of dry weight, while in 

 May it was less than the experimental error. Thus it appears that as 

 the dry season advances, Encelia loses more water by evaporation than 

 it absorbs by its roots until a certain minimum water-content is reached, 

 after which the relation between outgo and intake remains around 

 unity. In the meantime, some of the larger leaves die and the younger 

 ones obtain a very heavy coating of hairs. But in addition to this, a 

 viscous brown liquid is formed in the vessels which in May quickly 

 oozes from even a slight injury, while in January no such substance is 

 evident, even when the branches are severed. 



