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1906] SPALDING—ABSORPTION OF WATER BY LEAVES 263 
through their leaves, but the woody species growing in the arid 
territory of the southwestern United States are so different in their 
habits and in their environmental relations as to preclude the settle- 
ment of the question for them, even within the bounds of probabil- 
ity, in any other way than by direct observation and experiment; 
in fact they are found, as regards-leaf absorption, to differ widely 
among themselves. The object of the present paper, therefore, 
is the presentation of such facts as have been determined for a lim- 
ited number of species indigenous to southern Arizona. 
In this region existing physical conditions give to the question 
special interest. As is well known, precipitation is meager, except 
at relatively high altitudes, and is distributed throughout the year, 
with no distinctively rainy season. The rainfall, moreover, is 
extremely uncertain, and for months at a time is often so slight 
that it does not wet the soil for more than a few centimeters, an 
amount of precipitation likely to be of very little positive advan- 
tage as far as root absorption is concerned. Under such circum- 
stances, in which delicate adjustment is the condition of survival, 
it would seem that plants capable of leaf absorption might have a 
distinct advantage in times of prolonged drouth, during which occa- 
sional showers occur which are too light to penetrate the soil. As 
will be seen, however, only a limited number of species appear to 
enjoy this advantage to an appreciable extent. 
Nearly all of the species selected for investigation grow in the 
immediate vicinity of the Desert Laboratory. A single one, Hola- 
_cantha Emoryi, which seems not to be indigenous here, was obtained 
from the grounds of the University of Arizona. The following 
classification of the plants employed into biological groups is pro- 
Visional, but will serve to direct attention to the very diverse eco- 
logical history of the species now growing together in this region. 
BIOLOGICAL CLASSIFICATION OF PLANTS STUDIED. 
I. Shrubs, with relatively slight modification of form and struc- 
ture, their habits plainly indicating mesophytic origin. Celtis, 
Covillea, Lycium. 
II. Shrubs or small trees, more conspicuously modified, but 
retaining manifest traces of mesophytic habits. Parkinsonia, Pro- 
sopis, Acacia. 
