CHAPTER V. 
THE ORIGIN OF DESERT FLORAS.' 
The climatic and physiographic features which characterize deserts 
comprise combinations of meteoric and orographic factors to produce a 
rainfall markedly less than the possible evaporation, low relative humid- 
ity, comparatively small vertical increase in soil-moisture, low humus 
content of the soil, undeveloped surface drainage resulting in restricted 
areas highly charged with salts, comparatively great diurnal variations 
in both soil and air temperatures, and marked effects of wind movement 
and erosion on the surface. Many of the plants and animals of such 
regions exhibit distinctive habits and specialized structures articulating 
with the conditions of the limiting environmental factors enumerated. 
It is not to be taken for granted, however, that all of the organisms 
native or resident within a region properly designated as a desert show 
marked xerophilous qualities. On the contrary, it is quite possible 
that species suitable by form and structure for existence under conditions 
furnished by regions of great precipitation may find wide distribution in 
arid areas. This occurs, however, only when the scanty precipitation 
comes within a limited part of the year, giving a rainy season, or period 
of maximum precipitation, in which such forms may carry out their 
entire cycle of activity and then pass into dormancy, remaining qutes- 
cent in the form of seeds or modified shoots during the intervening drier 
seasons. It is to be noted, however, that such forms have peculiar 
rhythms and respond to changes in moisture and temperature in a man- 
ner not common to mesophytic species. 
A number of forms are to be found in every desert, having structural 
features which give them a distinct aspect, and it is to these that refer- 
ence is usually made in the characterization of the flora of any arid region. 
Such plants have many physiological capacities definitely suitable for 
activity during the drier seasons, and may in fact remain inactive in 
the periods furnishing conditions suitable for the activity of mesophilous 
forms. It is the origination of the qualities and characters which dis- 
tinguish these species that invites attention in the present connection. 
The first step in such an inquiry would naturally consist in going back 
over the paleobotanical records in the search for fossils of species, which 
might show evidences of having lived in an arid climate in previous 
epochs. Existing information as to the contour and profiles of ancient 
land masses gives every justification for the belief that extended areas 


1 Prepared by request and contributed by Dr. D. T. MacDougal, Director of Botani- 
cal Research, Carnegie Institution of Washington. 
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