76 MISSOURI BOTANICAL GARDEN BULLETIN 
amount of plant surface exposed to the atmosphere from 
which evaporation may take place, and has resulted in the 
cacti in the almost complete suppression of leaves, the thick, 
fleshy stems which remain serving to combine maximum 
bulk with a minimum of surface exposure. Even in those 
cases where leaves appear, they do not remotely resemble 
those of our ordinary plants and in most cases are small and 
scale-like and soon wither and fall away. 
A further safeguard against excessive transpiration exists 
in the relatively impervious layer which covers the entire 
surface of desert succulents. Although this mantle is inter- 
rupted at intervals by breathing pores, or stomata, it is other- 
wise quite impervious to water. It has been shown by 
experiment that a melon-cactus (Echinocactus), an almost 
globose leafless form in which the reduction of surface has 
proceeded to an extreme degree, transpires only about one 
five-thousandth as much water as a plant of equal weight 
of the Dutchman’s pipe (Aristolochia Sipho), a climbing 
plant bearing an abundance of thin, broad leaves and there- 
fore exposing a relatively enormous transpiring surface. 
These striking adjustments to the arid conditions of 
deserts are well illustrated in the specimens in the Garden. 
In the accompanying plate some of the cacti in the collec- 
tion are shown and it will be observed that in nearly all of 
them great bulk is combined with a minimum exposure of 
surface. Among others in the Garden may be mentioned the 
cylindrical-stemmed opuntias and the flat-stemmed species 
(“prickly pears,” or “Indian figs”) of the same genus. In 
these the stems are considerably enlarged, due to the presence 
of a great deal of water-storage tissue, whereas the leaves are 
much reduced, awl-shaped bodies which soon wither and fall. 
The members of the genus Cereus have developed a com- 
paratively large columnar stem which js leafless, or, in rare 
instances, provided with a few rudimentary leaves. The 
species of Echinocactus (hedgehog cactus) have much 
shorter, and proportionately thicker, stems than those of 
Cereus, forming spheroidal plants entirely destitute of leaves. 
Similar globose forms are to be found in the genus Mammil- 
laria, native of the United States, Mexico, and the West 
Indies, in Melocactus, indigenous to the West Indies and 
Central America, and in Echinopsis, at home in South 
America. 
Cacti are strictly American plants, growing in the arid 
or semi-arid regions of both continents from the southern 
part of British America to Argentine and Paraguay. For 
the most part they are of little economic value, sithouati 
