WATER RELATIONS OF DESERT PLANTS, 87 
tigations of Spalding (Biological Relations of Desert Shrubs, Botanical 
Gazette, 1906, vol. 41, p. 262) show that xerophytic types are the more 
unfitted for this purpose the more widely do they differ from those inhabit- 
ing moister regions. The chief specializations, then, of the plants of the 
desert are concerned with soil moisture, checks on loss of water, and the 
development of storage organs which would conserve a surplus accumu- 
lated during a period of much precipitation. 
SOIL RELATIONS OF DESERT PLANTS. 
The soils of deserts naturally present a wide range of physical charac- 
ter and chemical composition. In regard to the latter feature it is to be 
pointed out that such soils offer a small proportion of organic matter or 
humus, and that in many places certain saline compounds and alkaline 
salts are present in the surface layers, owing to defective drainage or lack 
of the leaching effect of precipitation. In such highly charged soils the 
specially adapted halophytic forms also characteristic of sea-beaches find 
a foothold. 
_ In all other soils in which clay, loam, sand, or rocks predominate the 
feature which has the greatest determining influence is that of the amount 
and disposition of the moisture. Many striking dispositions of the root- 
systems are being discovered which can only be correlated with the mois- 
ture factor. 
_ Water being so conspicuously lacking on the surface of a desert, the 
opinion finds favor that the roots of plants established there must have 
enormous penetration and reach down to a supposititious supply of water 
far below the surface. If such were the case these organs would in 
some instances need to penetrate several hundred feet, because after a 
depth of a few inches is reached the amount present does not increase 
very rapidly. Deeply penetrating roots are found for the most part in 
shrubs growing in loam or alluvium in bolsons or valleys where a supply of 
water is to be found within a reasonable distance of the surface, and the 
increasing amount coming with depth must act as a stimulating factor 
both in the growth and directive action of the plant. Thus Coville 
reports an observation of a main root of a Prosopis juliflora near the 
streamway of the Amargosa River in California which had a length of 
about 50 feet. A similar exaggerated vertical development is to be 
remarked especially in the seedlings of species inhabiting localities in 
which the supply of moisture lies deep. The date-palm makes an initial 
root of great length, as well as Welwitschia and numerous other forms. 
In the analysis of the meaning of any root-system it must be borne 
in mind that these organs have a dual function—that of anchorage and of 
absorption—while in some instances storage facilities are afforded both 
for water and for food material or surplus starch or sugar. 
