198 BOTANICAL GAZETTE [September 



valley), 1.9 per cent; sandy clay on the mesa, 3.9-10 per cent; 

 sandy clay in May on the mesa, 4 . 8-7 . 2 per cent. 



CAPACITY FOR HOLDING MOISTURE 



Some determinations were made to determine the capacity for 

 holding moisture, following the method used by Livingston, 5 

 with the following results. In the first column is shown the per 

 cent of water absorbed in proportion to the dry weight of the soil; 

 while in the second column the per cent of water is calculated in 

 terms of "wet volume," that is, the volume of the dirt when allowed 

 to settle under water. There is practically no humus in any of 

 the mesa soil. In the pinon association there is a little humus, 

 in the yellow pine association more, while in the Douglas spruce 

 there is abundant humus. 



Open mesa (Gutierrezia association) 23 .8 per cent 37 .3 per cent 



Bigelovia association (edge of mesa) where 



Bigelovia was most luxuriant 21.4 35.4 



Hymenatherum society of the association. . . .12.7 25.8 



WIND 



A factor influencing the evaporation from plants is wind. 



autumn 



calm 



rms 



man 



walking against it. These violent winds plants must 



may 



the mesa. The prevailing direction of the wind is 



south and southwest. This seems 



presence of sand dunes on the western edge of the valley and their 



absence on the eastern side. 



LIGHT 



In this clear atmosphere the illumination is of course intense 

 and very annoying to the traveler in summer. Concerning the 

 percentage of cloudiness Magnusson presents the following aver- 



s Livingston, B. E., Relation of desert plants to soil moisture. Bot. Gaz. 50: 

 241-256. 1910. 



