22 THE RELATION OF DESERT PLANTS TO 



the experiment of Jensen and the author (1904) upon the relation of 

 size of soil particles to plant growth. This sand has a water capacity 

 of about 46 per cent by dry volume. 



In these experiments the osmometers were filled with a solution of 

 cane sugar having a concentration of 2 gram molecules per liter. Rub- 

 ber stoppers with two perforations were used and a thermometer was 

 inserted in each cell beside the glass tube, so that the temperature of 

 the solution could be recorded with the readings on the height of the 

 column, and corrections could be made for temperature variations. 



It was found that the cell failed to absorb water from sands with 

 a water content of 1.5 per cent by volume; that neither absorption nor 

 water loss occurred in a sand of 5 per cent, and that absorption took 

 place from those of 7.5 and 10 per cent. It thus appears that the force 

 by which water is held in the 5 per cent sand is about equal to that of a 

 2-molecular cane-sugar solution, or at least 72 atmospheres. 



While the experiments with this form of ' ' artificial root hair ' ' have 

 not been carried far enough to justify any theoretical interpretation of 

 the results obtained, enough has been done to show that this method 

 offers a very valuable means for quantitative studies of the mechanics 

 of root absorption. It is hoped that further work may be done along 

 this line. A comparison of the results obtained upon the same soil by 

 this means and by means of the artificial root hair of Briggs and McCall 

 (1904) should throw light upon both the tenacity with which moisture 

 is held by a soil and the rate of movement of soil water. 



SUPPLY OF WATER TO THE SOIL. 



Situated about 80 meters above the Santa Cruz sandwash, Tuma- 

 moc Hill must receive all of its natural water from precipitation. The 

 annual precipitation here is practically the same as that at Tucson, for 

 which station records are available. These records, for fifteen years, 

 as given by Coville and MacDougal (1903, pp. 26, 27), show a mean 

 annual precipitation of 30.10 cm. (11.74 inches), which is distributed 

 mainly in two rainy seasons one in winter and early spring and one in 

 midsummer. This is shown clearly in Tables II and III, the first of 

 which presents mean monthly precipitations and the second the actual 

 record of precipitation at the Laboratory from May 11 to December 31, 

 1904. The data are for the 24 hours ending 8 a. m. on the date given. 

 For curves of the annual precipitation and average temperature at 

 Tucson, the reader is referred to Cannon (1905). Dr. Cannon has 

 kindly furnished the author with the data for Table III. 



