144 THE WONDER OF LIFE 



conditions is exhibited by desert plants which store water. 

 They have a relatively large root-system which enables 

 them to make the most of any available supply. F. V. 

 Coville found in the Mohave Desert, California, a branching 

 cactus {Opuntia echinocarpa), 19 inches high, which had a 

 network of roots extending over an area 18 feet in diameter. 

 These roots were 2 to 4 inches below the surface, suited 

 therefore for utilizing a downpour. Some desert plants send 

 their roots deep, and Professor R. H. Forbes has described 

 an acacia of Arizona which has a double root-system, one 

 for absorption near the surface, and the other for searching 

 deeply. 



The collecting surface is great, and the losing green surface 

 is small ; the whole plant, as in many cactuses, may be 

 like a ball or barrel and without leaves. The structure of 

 the cuticle and even of the transpiration pores is adapted 

 to lose as little as possible, and the interior of the plant 

 consists chiefly of water-storage cells, so that as much as 

 96 per cent, of water can be collected. The plant becomes a 

 tank and the water is often quite drinkable. A Barrel- 

 Cactus or Bisnago (Echinocactus emoryi) studied by CoviUe 

 yielded 3 quarts of water from about 8 inches of a plant 

 about a yard high and 20 inches in diameter. It may be 

 further noted that the Bisnago is effectively protected 

 against grazing animals by the impenetrable armour of 

 hooked and rigid spines, and another notable feature is the 

 fluted surface which allows it to expand and contract 

 without cracking. When we think of the root-system, 

 the leaflessness, the barrel-shape, the skin, the water-cells, 

 the spines, the fluting — we realize what a bundle of adapta- 

 tions this desert plant is, and many other slightly different 

 examples might be given. Although the Barrel-Cactus 



