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TEXTBOOK OF PLANT PHYSIOLOGY 



exhibiting a great diversity in their adaptation, one character is 

 common to all xerophytes; they exhibit a small transpiring surface. 

 This becomes especially conspicuous when compared with the com- 

 paratively large underground organs. Xerophytes are usually 

 herbaceous plants or dwarfy shrubs, in which the underground 

 parts are many times larger than the aerial organs (Fig. 75). 



Of the many common types of 

 xerophytes or desert plants, the most 

 peculiar are the cacti (Fig. 76). These 

 plants have thick fleshy stems, and 

 their leaves have lost their assimilative 

 function and have turned into spines. 

 Because of their shape, the cacti 

 combine a small surface with a con- 

 siderable volume. Usually they contain 

 large amounts of water which is spent 

 very slowly. Experiments have shown 

 that a cactus can thrive for several 

 months without an external water 

 supply. The root system of this plant 

 spreads near the surface of the soil. 

 During a rain, which occurs in every 

 desert, water is absorbed quickly and 

 collected into the stem. While in the 

 longer dry periods the small rootlets 

 die and only the thicker roots remain 

 and become covered with a corky layer. 

 The epidermis covering the stem of 

 the cacti has a very thick cuticle and 

 only a few stomata which are almost always closed. They 

 spend their water very scantily, therefore, even in the greatest 

 heat. But the absorption of carbon dioxide is thus also checked 

 and cacti are distinguished by slow growth. 



Physiologically related to the cacti are other typical fleshy 

 plants, in many of which water is stored in the leaves. Such are, 

 for instance, the aloes and agaves of the tropics (Fig. 143), and 

 species of Sedum (Fig. 77) and of Sempervivum of the temperate 

 zone. Such plants grow either in sand or on rocks, stone walls, 

 roofs, and in general where the thin soil layers often dry out. All 

 such fleshy plants are called succulents. 



Fig. 75. — Root system of Arte- 

 misia (after Weaver). 



