316 



FOUNDATIONS OF BOTANY 



with a thick epidermis, or else it must be able to revive 

 after being thoroughly dried. A few seed-plants and 

 many such spore-plants as lichens, Pleurococcus (Sect. 

 277), yeast, and some bacteria (Sect. 263), thrive just as 

 well after remaining for some months 

 or years in a dried condition as they 

 did before drying. A good illustration 

 of this fact as regards yeast is found in 

 the use of dried yeast cakes, made of a 

 mixture of yeast and corn meal. These 

 will raise dough promptly when mixed 

 with it, even if they have been kept 

 dry for a year or more. 



386. Roots and Stems of Xerophytic 

 Seed-Plants. Some xerophytes have 

 roots which show no peculiarities of 

 form or structure, but many make special 

 provision for storing food and water in 

 their roots. Such roots are fleshy and 

 often, as in Harpagophytum (Fig. 223), 

 are of great size compared with the 

 portion of the plant above the ground. 

 Xerophytic stems are frequently very 

 thick in proportion to their length, 

 sometimes even globular, and they 

 commonly contain large amounts of water. In leafless 

 plants, like the cacti, the surface for transpiration is much 

 less than that offered by leafy plants. Many species which 

 bear leaves shed most of them at the beginning of the dry 

 season, and some remain thus in a half dormant condi- 

 tion for long periods, as is the case with many Euphorbias 



FIG. 223. Harpago- 

 phytum, a South 

 African Xerophyte. 



