LEAVES 635 



It has been shown experimentally that a leaf with a surface layer of 

 water becomes less heated on exposure to light than does the same leaf 

 without such a layer. 



Peripheral water tissues. Peripheral water tissues commonly are associated with 

 expanded dorsiventral leaves and are especially characteristic of the tropical rain 

 forest, where they are found particularly in epiphytes (such as the orchids and bro- 

 melias), whose water supply is more limited than that of the trees on which they 

 grow, because they can absorb water only when it is in contact in liquid form with 

 their aerial organs; similar tissues occur also in the leaves of various trees, as in 

 Ficus, and even in those of such mesophytes as the banana. Except in the epiphytes, 

 the advantage of water tissue in the rain forest is not obvious, though it has been 

 suggested that the water mantle is of value, even there, as a means of reducing the 

 intensity of the light and of lessening the occasionally high transpiration. Even these 

 somewhat doubtful advantages must be lacking in the case of Begonia and Pepe- 

 romia, plants with pronounced leaf succulence that often live in the dense shade of 

 the forest bottom. Such plants may have immigrated to the forest from a former 

 xerophytic habitat, retaining their xerophytic structures ; or their xerophytism may be 

 inherent and quite unrelated to external conditions. That in Begonia some species, 

 at least, are essentially, rather than apparently, xerophytic is shown by their 

 frequent cultivation in houses in relatively xerophytic conditions, while most of their 

 associates of the rain forest (such as the filmy ferns) require the most humid of 

 conditions for successful cultivation. It is difficult to imagine any advantage in suc- 

 culent leaves in the depths of the rain forest. 



The significance of succulence in extreme habitats. In the most ex- 

 treme desert xerophytes and in the plants of salt marshes and 

 alkali regions, equilateral leaves with centrally placed water tissue are 

 very characteristic. There is little doubt that succulence represents 

 the culmination of xerophytic characters and that the equilateral leaf 

 with central water tissue represenls the most xerophytic of leaf forms. 

 Salt deserts are the most unfavorable of habitats, since with the most 

 excessive transpiration there are combined the poorest conditions for 

 absorption, by reason of the high concentration of the soil solutions. 

 Among the higher plants the forms which penetrate the farthest into 

 these barren regions are the succulents. In conditions where existence 

 is impossible for the sagebrush with its hairy leaves, or for the creosote 

 bush with its varnished leaves, or even for the leafless cacti with their great 

 quantities of accumulated water protected by cutin, there flourish various 

 Chenopodiaceae (such as Salsola, Sarcobatus, or Suaeda) with their 

 fleshy equilateral leaves. Three features appear to be responsible for 

 the remarkable endurance of such leaves : the small evaporating surface, 

 which is the least of any leaves ; the large amount of water, which serves 



