no THE POPULAR SCIENCE MONTHLY. 



external enemies. The prickly pear, in fact, is a typical instance 

 of a desert plant, as the camel is a typical instance of a desert 

 animal. Each lays itself out to endure the long droughts of its 

 almost rainless habitat by drinking as much as it can when oppor- 

 tunity offers, hoarding up the superfluous water for future use, 

 and economizing evaporation by every means in its power. 



If you ask that convenient fiction, the Man in the Street, what 

 sort of plant a cactus is, he will probably tell you it is all leaf and 

 no stem, and each of the leaves grows out of the last one. When- 

 ever we set up the Man in the Street, however, you must have 

 noticed we do it in order to knock him down again like a nine-pin 

 next moment : and this particular instance is no exception to the 

 rule ; for the truth is that a cactus is practically all stem and no 

 leaves, what looks like a leaf being really a branch sticking out 

 at an angle. The true leaves, if there are any, are reduced to 

 mere spines or prickles on the surface, while the branches, in the 

 prickly -pear and many of the ornamental hot-house cactuses, are 

 flattened out like a leaf to perform foliar functions. In most 

 plants, to put it simply, the leaves are the mouths and stomachs 

 of the organism ; their thin and flattened blades are spread out 

 horizontally in a wide expanse, covered with tiny throats and lips 

 which suck in carbonic acid from the surrounding air, and disin- 

 tegrate it in their own cells under the influence of sunlight. In 

 the prickly pears, on the contrary, it is the flattened stem and 

 branches which undertake this essential operation in the life of 

 the plant the sucking-in of carbon and giving-out of oxygen, 

 which is to the vegetable exactly what the eating and digesting 

 of food is to the animal organism. In their old age, however, 

 the stems of the prickly pear display their true character by be- 

 coming woody in texture and losing their articulated leaf-like 

 appearance. 



Everything on this earth can best be understood by investi- 

 gating the history of its origin and development, and in order to 

 understand this curious reversal of the ordinary rule in the cactus 

 tribe we must look at the circumstances under which the race was 

 evolved in the howling waste of American deserts. (All deserts 

 have a prescriptive right to howl, and I wouldn't for worlds de- 

 prive them of the privilege.) Some familiar analogies will help 

 us to see the utility of this arrangement. Everybody knows our 

 common English stone-crops or if he doesn't he ought to, for 

 they are pretty and ubiquitous. Now, stone-crops grow for the 

 most part in chinks of the rock or thirsty, sandy soil ; they are es- 

 sentially plants of very dry positions. Hence they have thick and 

 succulent little stems and leaves, which merge into one another 

 by imperceptible gradations. All parts of the plant alike are 

 stumpy, green, and cylindrical. If you squash them with your 



