go BOTANICAL FEATURES OF NORTH AMERICAN DESERTS. 
absorbing organs and the foliar or transpiratory surfaces. Roots like 
those of the Echinocactus or of the small annuals must be exposed to 
temperatures identical with or differing but little from those of the shoot. 
Deeply penetrating roots, on the other hand, may often have a tempera- 
ture as much as 40° or even 60° F. from the shoots to which the stream of 
soil solution is sent. 
In consequence of the great excess of possible evaporation over the 
precipitation in desert regions, the amount and condition of the moisture 
in the soil must be largely dependent upon the physical properties of the 
principal soil constituents and of the underlying geological formation. 
This excessive evaporation may in itself become a factor in the laying 
down and structure of strata near the surface, and also indirectly of 
formations at considerable depth. Chief among these is the “‘caliche”’ of 
Southwestern deserts, which is probably duplicated in the lime deposits 
of the deserts of southern Africa and of other parts of the world. The 
evaporating power of the exceedingly dry air of the desert removes the 
moisture from the surface layers of the soil much more rapidly than it 
may be supplied by movement through the films surrounding the soil 
particles from below; consequently the surface layers soon become 
“air-dry” and the hypothetical capillary columns end beneath, retreating 
to a depth where the evaporation hindered by the blanket of drysoil above 
is balanced by the supply. Such an equilibrium is generally attained at 
a depth less than a yard beneath the surface. The slowly moving but 
constant stream of water charged with lime therefore evaporates and 
necessarily leaves its lime content at this depth, and in the course of 
centuries a ‘‘caliche’”’ formation of considerable thickness may be formed ; 
this in itself is a factor of great influence upon the habits of the plants 
which root in the soil above it. 
Recently methods have been developed for the intensification of the 
action of the air-dry layers by pulverizing it in such manner as to form 
a dust mulch in what is known as “‘dry-land farming.’”’ The retarding 
effect of a dry surface layer is well illustrated in the gypsum sands in 
New Mexico. The action of the sun heats and dries the surface of the 
dunes to a snowy whiteness, and this dry layer has a thickness of a few 
inches only. It may be removed readily and layers moist and cool to 
the hand uncovered in the most arid season. By reason of this supply 
such plants as poplars, which are able to endure the highly charged soil 
solutions, but which require fairly abundant moisture, grow readily. 
The air-dry layer is in part responsible for the long periods of dor- 
mancy of the bulbs, tubers, and seeds of many desert species. Such 
structures with the protoplasm in a resting condition, protected by heavy 
outer cellulose coats, may lie inert for indefinite periods in the loose layers 
of the dust blanket of arid regions. 
