282 BOTANICAL GAZETTE [APRIL 
SUMMARY. 
From the preceding observations and experiments, in which 
woody plants were chiefly employed, it has been seen that certain 
species of desert plants of southern Arizona absorb water presented 
to their leaves and internodes, while others do not. The species 
represented in the vicinity of the Desert Botanical Laboratory may 
be divided into several biological groups, based primarily on the 
water relation, of which leaf absorption is a phase. Thus, in the 
first group, including shrubs, which retain well marked mesophytic 
tendencies, leaf absorption is characteristic. Members of the 
second group, more distinctively xerophytic in various structural 
particulars, are incapable of leaf absorption during their period 
of normal activity. The third group, decidedly xerophytic, but 
including species of widely different structure and habits, exhibits 
corresponding differences in regard to subaerial absorption, which 
takes place in some of its representatives and not in others. The 
fourth group, including cacti which are assumed to represent the 
extreme type of xerophytes, also exhibits interesting differences 
in size and structure of the tubercles by means of which water is 
absorbed. Finally, members of a provisional fifth group, which 
in habit and structure are nearer than any other others to the meso- 
phytes of moist temperate regions, absorb water largely, but very 
quickly give it up again. 
It may be doubted, perhaps, whether this classification, based 
on biological relations, has in itself any permanent value, but mean- 
time it serves to express and emphasize what is apparently no mere 
theoretical conception, but a simple historical fact, namely, that 
differences of habit on the part of these desert plants, as well as 
the structural adaptations with which they are correlated, have 
become established step by step together, during the long period of 
geographical changes through which the land they now occupy 
has been passing. A discussion of the physiological significance 
of the facts which have been brought out does not fall within the 
province of this pay er. 
DESERT BOTANICAL LABORATORY, 
son, Arizona. 
