366 



PLANT PHYSIOLOGY 



''xeroph3rtes." Plants characteristic of perpetually moist habi- 

 tats, near bodies of water or in tropical rain forests, are called 

 ''hydrophytes." The majority of plants of temperate climates 

 and of moderately moist habitats are of an intermediate type, 

 the "mesophytes." A large majority of crop plants belong to 

 this group. 



Since none of these ecological types are represented by a 

 homogeneous group, they cannot be characterized by a few 



definite physiological features. This 

 is especially true of the xerophytes, 

 plants that have adapted themselves 

 to very severe habitats. But though 

 exhibiting a great diversity in their 

 adaptation, one characteristic is com- 

 mon to all xerophytes: They have a 

 small transpiring surface. This feature 

 becomes especially conspicuous when 

 compared with the relatively 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. 114). 



Of the many common types of 

 xerophytes, or desert plants, the most 

 peculiar are the cacti (Fig. 115). 

 They have thick fleshy stems, and 

 their leaves have lost the assimilative 

 Fig. 114.— Root system of Ar/e- function and have turned into spines. 



mieia {after Weaver) . -p. /• , i • i , i 



Because ot their shape, the cacti 

 combine a small surface with a considerable volume. Usually 

 they contain large amounts of water, which is used very slowly. 

 Experiments have shown that a cactus can thrive for several 

 months without any 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. During 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 stems 

 of the cacti has a very thick cuticle and only a few stomata, which 



