464 



ECOLOGICAL GROUPS 



oases of the Sahara, where the soil is moist from the presence 

 of springs, though rains are almost unknown, or the houseleeks 

 and stonecrops found in many gardens, the so-called Spanish 

 moss (Plate III), and lichens (Figs. 226, 227), all of which grow 

 most rapidly in moist air, but cling to bare rocks and trunks 

 of trees, from which they get no water. 



It is important to notice that many xero- 

 phytes only economize water ivhen forced to 

 do so. With an abundant supply of water 

 they may transpire almost or quite as much 

 as mesophytes. But a drought which 

 would kill the latter would only cause the 

 xerophytes to close their stomata and greatly 

 lessen transpiration. A xerophyte must be 

 capable of storing water and transpiring 

 very slowly, like cactuses, aloes, stonecrops, 

 and such fleshy plants with a thick epider- 

 mis, or else it must be able to revive after 

 being thoroughly dried. 



439. 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 



FIG. 358. Ilarpago- are flesh y and often ' as in " Harpagopliytnm 

 phytum, a South (Fig. 358), are of great size compared with 

 African xerophyte the p Ort i on o f the plant above the ground. 

 After Schimper Xerophytic stems are frequently very thick 

 in proportion to their length, sometimes even globular (Fig. 50), 

 and they commonly contain large amounts of water. In leaf- 

 less plants, like the cactuses, 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 condition 

 for long periods, as is the case with many Euphorbias (Fig. 318). 



