16 THE RELATION OF DESERT PLANTS TO 



the case of sandy soils lying above the level of underground water, the 

 water of a heavy shower percolates rapidly and is drained away below, 

 leaving but a small amount in the upper layers. But in clay soils a very 

 much larger amount of water is held by capillarity and fails to drain 

 away. For this reason, in regions where long periods elapse between 

 rains, those upland soils which are more clayey in their nature are uni- 

 formly better adapted to plant growth during the periods of drought 

 than are the more sandy ones which retain less water. For a discussion 

 of this subject in reference to more humid regions the reader is referred 

 to publications by Warming (1902, p. 55), Schimper (1898, p. 94), 

 Livingston (1905), and Livingston and Jensen (1904). 



As should be expected from its nature, the clay of Tumamoc Hill 

 has a high water capacity or retaining power. Determination of this 

 property was made by the usual method. A tin cylinder, 13 cm. high 

 and 8 cm. in diameter, with a perforated tin bottom covered exter- 

 nally with cloth, was used for this purpose. This vessel was partially 

 filled with soil, tamped in, and the whole was weighed. Then the cylin- 

 der was placed upright in water, so that the surface of the latter was 

 somewhat above that of the soil within, and water was poured in 

 above the soil until it stood several centimeters deep above the latter. 

 When the soil was thoroughly saturated the cylinder was removed and 

 allowed to drain until water ceased to flow out through its bottom, after 

 which a second weighing was made. Finally, the volume of the soil 

 when allowed to settle under water was determined, and the difference 

 between the two weights taken as the amount of water retained by the 

 soil. This was calculated in percentage of the wet volume of soil used, 

 of the dry volume tamped, and of the dry volume not tamped. An 

 average of five such determinations gave the amount of water retained 

 by this soil as 40.9 per cent of its wet volume, 59.8 per cent of its dry 

 volume tamped into the cylinder, or 51.8 per cent of its dry volume not 

 tamped. This water capacity, or retaining power, is very high, although 

 it does not reach that possessed by some of the heavy clays of Mich- 

 igan which the author has dealt with. One sample from that State had 

 a retaining power of 62.5 per cent of its dry volume untamped. 



From the determination just given it is evident that the soil under 

 consideration retains, and prevents from draining away below, an 

 enormous amount of water, and to this fact is probably due the prev- 

 alence on the hill of a number of plant forms which derive most of their 

 water from near the surface. Cereus and Echinocactus are examples 

 of these. It is clearly shown by the work of Mrs. E. S. Spalding (1905) 

 on Cereus that this plant derives most of its storage water from the 

 surface layers when these have a high moisture content following a 



