SOIL MOISTURE AND TO EVAPORATION. 9 



lost their leaves and stood as groups of gray, spiny wands; many others 

 were still green and appeared healthy, but practically all had scattered 

 their seeds. A single belated cluster of ocotillo flowers was found near 

 the top of the mountain on July 7. The trees of palo verde (Parkinsonia 

 microphylla) near the Laboratory had very largely lost their leaves, 

 thus also showing the effect of drought. Of the smaller plants, Encelia 

 farinosa still held its own as far as foliage was concerned, and a small 

 red mallow, Sphaeralcea pedata, together with a prostrate Euphorbia, 

 probaby E. capitellata Eng. , of somewhat the aspect of E. polygonifolia 

 of the East, were producing flowers and seemed perfectly vigorous. 



The extreme dryness exhibited by soil, air, and vegetation, together 

 with the fact that the summer season of rains was rapidly approaching, 

 made it seem very important to take up immediately the question of the 

 actual amount of water contained by the soil at that time. Accord- 

 ingly a great number of diggings were made on the slopes of the hill 

 and on its top around the Laboratory, care being taken not to locate 

 any of these within possible reach either of the water tank, where 

 small amounts of water were usually escaping, or of the outlet of the 

 waste pipe below the building. Samples of soil were collected in this 

 manner from various depths and immediately placed in glass vials, 

 which were tightly stoppered and weighed. The samples were then 

 emptied into Stender dishes, of the form used for staining microscopic 

 preparations, and, in default of a suitable drying oven, left open in the 

 laboratory 5 to 15 days, being stirred occasionally to hasten evapora- 

 tion. When these had ceased to lose water they were returned to stop- 

 pered vials and their weight was again recorded. After the author's 

 return to the University of Chicago these samples were again weighed, 

 dried thoroughly in an oven at a temperature of from 105 to 110 C., 

 and the amount of water thus lost was added to that which had been 

 lost in air-drying at the Desert Laboratory. The amount of water 

 present in the original samples was computed on the basis of volume 

 per cent. While for comparisons between different samples of the same 

 soil the water content may be determined in percentage of the dry 

 weight of the soil, this method fails to have even a practical value 

 when soils of different specific gravities are dealt with. This point, 

 while it has been mentioned by Whitney and Hosmer (1897, p. 7) and 

 others, has never been adequately emphasized from the standpoint of 

 plant physiology. From this standpoint the interesting questions are, 

 first, how much water is within reach of the plant, and, second, how 

 much of this water can be absorbed by the roots? In the answer to the 

 first question the specific gravity of the soil can play but a minor part, 

 the main factor being the volume of soil drawn upon by the roots and 



