1 6 ROOT HABITS OF DESERT PLANTS. 



precipitation, but the total penetration naturally does not exceed the 

 thickness of the adobe, that is, 50 cm. more or less, usually less. 



The studies on the movements of the moisture of the soil, on which this 

 account is mainly based, comprise observations from October 3, 1907, to 

 April II, 1908, and include, therefore, only the dry autumn and dry early 

 winter, the winter rainy season and the earh^ spring. Studies on the condi- 

 tions for the remainder of the year are not now available. The special 

 studies were made on soils at two depths, namely, 15 and 30 cm. The soil 

 was removed at frequent intervals, placed in stoppered bottles, and dried 

 in an oven at the laboratory. 



The following summary gives the water movements during the period 

 under consideration, together with the rainfall recorded on Tumamoc Hill 

 at the time: 



From October until the latter part of January 1.68 inches of rain fell, 

 and the curves of soil moisture at this time were fairly flat; that for the 

 deeper soils fluctuated less than that for the shallow soils. Between 

 January 13 and February 11, 1.54 inches of rain were reported at the 

 laboratory. The soils on the first of February were the driest of the period 

 under observation. At the 15 cm. depth they contained 15 per cent water ; 

 at the depth of 30 cm., 14.5 per cent (dry weight). During the period 

 between February i and February 1 1 , when the next soil observation was 

 made, 1.49 inches of rain fell. On February 11 the amount of water in 

 the soil at the 15 cm. depth was 34.8 per cent, which was the largest amount 

 at that depth found at any time during the season. At this time there was 

 19. 1 percent water at the 30 cm. depth. Between February 1 1 and March 23, 

 0.87 inches of rain fell, in small amounts. During this period the amount 

 of water in the soil at the higher level had fallen to 18.9 per cent, but the 

 water content at the depth of 30 cm. had slightly increased. 



Although the data for the year are not available to show it, it is fair to 

 assume that, as the dry summer approached, the soils at both depths 

 became more dry and reached their extreme desiccation in July when the 

 summer rains began. The per cent of water in the soils at the time is not 

 known, but observations on the soils at the laboratory made 4 years pre- 

 viously in July, gave the following results: The water content was 17.83 

 per cent at a depth of 30 to 40 cm. ; at a depth of 40 cm. it was 15.8 per 

 cent of the dry weight of the soil. At the same time, at the 15 cm. depth 

 the moisture content was only 9.1 per cent. The significance of these last 

 figures becomes apparent when it is recalled that the minimum moisture 

 content of this soil which can be available to plants not possessed of water- 

 storage organs is 10 per cent its dry weight. 



From these data, admittedly inadequate, we can understand some of 

 the characteristics of the environment and the conditions of growth of the 

 flora of Tumamoc Hill. The soil to a depth of 15 cm. probably does not 

 retain sufficient moisture for absorption by plants whose roots do not 



