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, Plewrococcus (Sect. 
277), yeast, and some bacteria (Sect. 2638), 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 
Fie, 223. — Harpago- ‘ : 2 ° 
phytum, a South thick in proportion to their length, 
African Xerophyte. 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 Luphorbias 
