94 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 
CLIMATIC CONDITIONS. 
The general climatic conditions which prevail in the region in which 
the Desert Laboratory is located are well known. They are, briefly 
stated, such as are necessarily characteristic of more or less elevated, 
semi-arid, continental areas in lower and middle latitudes. The annual 
precipitation is meager and irregular in amount, and this, with low rela- 
tive humidity, high rate of evaporation, wide diurnal range of tempera- 
ture, high winds, and intense insolation, presents a combination of trying 
conditions, to which, however, a large number of plant species have 
successfully adapted themselves. 
These conditions are, in general, characteristic of all the great desert 
regions of the globe, which, however, differ widely in important partic- 
ulars, the differences being reflected more or less distinctly in the habits 
and distribution of the plants inhabiting them. As an example, the 
distribution of rainfall in the desert region of the southwestern United 
States is essentially different from what it is in the Egyptian-Arabian 
deserts. In the latter the period of maximum rainfall and of greatest 
vegetative activity is in the winter, while in summer the vegetation is 
practically dormant, the time at which on the Laboratory domain the 
summer annuals are at the height of their vigorous growth. Such a 
state of affairs necessarily results in great differences, not only in the 
character of the summer vegetation, but also in periods of growth of 
perennials, and in structural and physiological relations. The studies of 
this subject at the Desert Laboratory by Cannon (1905) are highly sug- 
gestive and show the desirability of extended investigation. 
The lack in botanical literature of sufficiently detailed observation and 
experiment regarding local climatic conditions in their relation to the 
growth and distribution of plants suggests that, without attempting a 
discussion of the broad subject of desert climates, it will be desirable in 
this place to call attention to a limited number of recent records and 
studies looking toward more exact knowledge of local conditions and 
their influence ou the distribution of plants. 
RECORDS OF RAINFALL. 
Records of rainfall show that in this region the amount of precipita- 
tion is extremely variable from year to year in the same place, and that 
it often differs very considerably in places not widely remote from each 
other. In table 6 are given the records as taken at the Arizona Agri- 
cultural Experiment Station and at the Desert Laboratory, 3 miles 
distant and at approximately 200 feet greater altitude, for the period 
March, 1906, to May, 1907, inclusive. They show a fairly close corre- 
spondence for most storms, but it is also seen that summer rains espe- 
cially, even at this short distance, are often extremely irregular and 
unlike in amount of precipitation at the two points. As an example, 
