RESISTANCE OF PLANTS 



367 



are almost always closed. They lose their water very slowly, 

 even in the greatest heat. But 

 the absorption of carbon dioxide 

 is also checked, and cacti are 

 distinguished by slow growth. 



Physiologically related to the 

 cacti are other typical fleshy 

 plants, in many of which w^ater 

 is stored in the leaves. Such 

 are, for instance, the aloes and 

 agaves of the tropics (Fig. 129) 

 and species oi Sedum (Fig. 116) 

 and of Sempervivum of the tem- 

 perate 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." 



Quite different from these are 

 the thin-leaved xerophytes. 



To this type belong such plants Fig. 115. — Gigantic cacti in an Arizona 



as Alhagi camelorum, the wild " ^ 



alfalfa of the steppes, and some representatives of the Cucur- 

 hitaceae, like Citrullus colocynthis. Likewise, species of the 



sagebrush Artemisia, found in 

 plains and semideserts, closely 

 approach this group. These 

 plants have thin tender leaves, 

 which wilt rapidly when a 

 branch is cut from the plant. 

 The mtensity of transpiration 

 is very high in plants of this 

 type, especially in sunlight. 

 Even on very hot dry days, 

 they keep their stomata mde 

 open, losing considerable 

 amounts of water but assmiilat- 

 ing energetically. As a result 

 of this vigorous transpiration, xerophytes of this type are able to 



Fig. 116. — Sedum acre. 



