304 CARNEGIE INSTITUTION OF WASHINGTON. 



Slope-Exposure Studies, by F. E. Clements and Dolly Lutjeharms. 



The investigation of the striking differences in the vegetation of the north 

 and south slopes of Engelmann Canyon has been completed, and the results 

 are in process of being published in full, in addition to being abstracted in 

 the forthcoming paper on the phytometer method. 



The data for the physical factors show that these are much more intense 

 on the south-facing slope than upon the opposite one. Air temperature and 

 humidity exhibit the smallest differences between the two slopes, 2 and 6 

 per cent, respectively, while light intensity, evaporating power of the air, 

 and wind velocity are each 50 per cent greater on the south slope-exposure. 

 Soil temperature presents more variation between the slopes than does air 

 temperature — 32 per cent as contrasted with 2 per cent. In spite of the fact 

 that the rainfall is practically uniform, the soil moisture at all depths on the 

 south slope-exposure is approximately one-third less. 



The phytometers responded directly to the surrounding conditions, and 

 growth increment, with the exception of stem height, was greater on the south 

 exposure. These plants, with 14 per cent more leaf area, 10 per cent greater 

 stem diameter, and 25 per cent less stem height, were able to produce 75 per 

 cent more dry matter. The lesser stem height is believed to be a direct result 

 of the heavy transpiration, which was 82 per cent greater on the south 

 exposure. 



It is thus apparent that the factors which favor high transpiration, namely, 

 high wind velocity, high evaporating power of the air, and high soil temper- 

 ature, hkewise favor high photosynthetic activity. Light intensity is also 

 an important factor in the production of dry matter. Judging from the 

 responses of the phytometers, high wind velocity, high evaporating power 

 of the air, and low holard, especially the latter, seem to determine the open 

 aspect of the south slope-exposure. On the other hand, the denser, more 

 wooded character of the north slope is due to the reverse conditions — low 

 wind velocity, low evaporating power of the air, and high holard. 



The Water Cycle in Plants, by F. E. Clements and J. V. G. Loftfield. 



The emphasis during the summer has been placed on the plasmolysis 

 method for determining cell-sap concentrations. Although this has been 

 used by others almost wholly with plants having colored sap, it has been found 

 possible to employ it with reasonable certainty in all the plants studied. Most 

 of the work has been done with a selected strain of sunflower, grown at the 

 Alpine Laboratory in three factor stations, namely, in the open, in partial 

 shade, and in dense shade. A number of 24-hour series were made with these 

 plants, in which the stomatal opening and guard-cell and epidermal-cell 

 concentrations were measured at 3-hour intervals. Although both Iljin and 

 Wiggans found the concentration of epidermal-cell sap to remain constant 

 while that of the guard-cells varied, this did not prove to be the case in sun- 

 flower or in any of the other plants studied. At times the concentration of 

 the epidermal cells was very nearly constant, more often it was found to vary 

 appreciably, and in a few instances it varied more than in the guard-cells. 

 Usually, the concentration of the epidermal-cell sap rose after 10 a. m. and 

 continued to do so until after midnight, when it would start to fall. The 

 concentration within the guard cells rose sharply after daybreak, when 

 stomatal opening started, and showed the greatest departure from that of 



