300 THE INTERNAL WATER RELATIONS OF PLANTS 



such as sand dunes and beaches in regions of humid climate. The most con- 

 spicuous succulents of the American semi-desert regions mostly belong to the 

 cactus family {Cactaceae) . The other more important families of plants 

 which include a number of succulent species are the Euphorbiaceae, Liliaceaej 

 CrassuloceaCj Aizoaceae, and A?naryllidaccae. The succulents are a distinc- 

 tive group of plants not only in structure, but in metabolism (Spoehr, 191 9) 

 and water economy as well. Species of the succulent habit of growth are 

 able to survive dry periods because of the relatively large reserves of water 

 which accumulate in the inner tissues of the fleshy stems or (in some species) 

 in the fleshy leaves. A relatively thick cuticle and the fact that in many suc- 

 culents the stomates are generally open only at night are important factors 

 in permitting the conservation of water by such species. Many cacti can live 

 for months on this stored water even if entirely uprooted from the soil. The 

 recent vogue for some of the smaller species of cacti as house plants probably 

 has been favored by the capacity of such species for survival in the arid at- 

 mosphere of the average house. 



Neither the ephemcrals nor the succulents can be regarded as truly drought 

 resistant in the sense that their cells can endure a severe reduction in water 

 content for extended periods of time without injury. This is true only of 

 species which have been classed in the "drought enduring" group. One of 

 the most extreme examples of such a species among higher plants is the creosote 

 bush {Larrea tridentata Cav.) which is the dominant plant through large 

 areas of the semi-arid regions of the southwestern United States and northern 

 Mexico. This species carries the same set of leaves through both the wet 

 and dry seasons. During drought periods the water content of the leaves of 

 the creosote bush is sometimes less than 50 per cent of their dry weight (Run- 

 yon, 1936). The water contents of the leaves of most woody mesic species, 

 on the other hand, generally range between lOO and 300 per cent of their 

 dry weight. 



Some species of plants, including especially many mosses, lichens, and algae, 

 can be reduced to a virtually air dry condition during drought periods, yet 

 remain viable, and resume their life processes very quickly when they are 

 again provided with a supply of moisture. The seeds of many species are 

 drought resistant in this sense that they may be reduced to a nearly air dry 

 condition without losing their viability. 



All attempts to explain drought resistance of "drought-enduring" plants 

 upon a purely morphological basis have proved inadequate, although certam 

 structural features of plants undoubtedly aid in their survival in dry habitats. 

 Many xerophytes, for example, have extensive root systems in proportion to 

 their tops. Such root systems may efficiently tap a very considerable volume 



