74 MISSOURI BOTANICAL GARDEN BULLETIN 
culent forms, typified by the cacti, which are so characteristic 
a part of the American desert landscape. 
These plants distinguish themselves not only by their 
peculiar habitat, but also by their often strikingly grotesque 
forms, which have earned for them the appellation of “car- 
toons of the plant world.” In fact, these plants are so 
peculiar in their form and structure that they stand out 
prominently as a distinct group. Moreover, wherever desert 
conditions are encountered in America, plants of this type 
are almost invariably met with, and it becomes of interest, 
therefore, to inquire into the forces which are operative in 
these desert regions in bringing about such modifications 
and adjustments. 
A small annual rainfall (amounting in many of the arid 
and semi-arid regions of the southwest to not more than 
8-15 inches, whereas that in the vicinity of St. Louis amounts 
to about 37 inches) usually coupled with high temperatures 
and an excessive rate of evaporation, due to the dry, warm 
— are perhaps the most obvious characteristics of 
most desert regions. In attempting an analysis of plant life 
under these conditions it is at once evident that the ques- 
tion of water supply is of paramount importance. 
Generally speaking, practically all of the water absorbed 
by plants is taken up by the roots. In addition to absorb- 
ing water, the plant is constantly giving off, or transpiring, 
water vapor to the air from the aérial parts, notably from 
the leaves. The yee of this evaporation from the plant is 
governed, other things being equal, by the same factors 
which apply to evaporation from free water surfaces, namely, 
ppt eles’ area of water surface exposed, and frequency 
of change of the surrounding atmosphere. It is of import- 
ance, therefore, to call attention here to the fact that the 
question of water supply in plants naturally divides itself 
into two propositions, (1) the amount of water absorbed by 
the roots, and (2) the amount of water transpired, or con- 
a stated, the amount of absorbed water which is con- 
served. 
It is popularly supposed that the roots of desert plants are 
of great length and that they penetrate the ground to great 
depths in order to reach the water-containing strata during 
the long, dry seasons. While this may be true to a certain 
extent in the non-succulent desert plants, especially those 
of the Old-World desert regions, it is notably not true for the 
fleshy, succulent plants — the plants which are to be con- 
sidered in the present discussion. In these the roots, at 
least those which are actively concerned with water absorp- 
