196 
girth at intervals of a few hours, when the rapidity of the process 
will remind one of absorption by a piece of sponge 
It must be mentioned, however, that there are some cactuses 
which actually live in moist forests and thrive in a humid atmos- 
phere. Many of these moist-climate species, if not most of 
them, are epiphytic, growing on the trunks or branches of trees. 
Owing chiefly to the peculiar life-habits mentioned, the cactuses 
have developed ae forms, hare age snide: among 
them itea 
selves, constitu 
ost important 
of these seuiceural changes is the . i the stem of 
various massive forms, composed largely of soft tissue. Next in 
importance and ina e is the loss of the leaves, and a 
corresponding modification of the outer bark, by which it 
assumes the duties so originally ne to the leaves. It 
is evident that the evaporation from ordinary leaves, in the torrid 
atmosphere of the desert, would quickly result in the loss of 
stored water—long before the new annual supply became avail- 
able; thus, under that law of life that declares that that which 
a r animal needs to enable it to successfully encounter 
the conditions of its environment, it shall secure, the cactus 
loses its leaves, at least as to their foliaceous form, and the 
epidermis of the stem is so modified as to assume wholly their 
duties, instead of iad aiding a little in their performance, as 
is customary among other plants. Their surface thus becomes 
smooth, green ay fen ike. 
The leaves are not wholly lost, but are converted into spines, 
which often assume very strange and peculiar forms. That these 
changes are the result of environment, is evidenced by the fact 
that in various other families, especially the spurge or castor-oil 
family, almost exactly similar forms occur among the members 
which live under similar conditions. 
The loss of moisture by evaporation is not the only danger to 
which plants of the desert are subjected. Their rich stores of 
water and of soft and digestible tissue, make them eagerly sought 
by foraging animals, against which they are protected by the 
formidable spines which I have described and illustrated. Some 
few possess no spines, having lost, their leaves altogether. These 
