Chap. I] 



WATER 



19 



The greatest possible expansion of the transpiring surface is the most 

 general characteristic of hygrophytes. How important this may be has been 

 shown by comparative experiments made 

 by Noll, on a large-leaved hygrophilous 

 plant, Aristolochia Sipho 1 , and on a sphe- 

 roidal Echinocactus. His observations 

 showedthat,forequal weights, the Echino- 

 cactus possessed a surface 300 times 

 smaller than the Aristolochia. Were 

 it merely a question of the size of surface 

 exposed, the ratio of the transpiration of 

 the two plants would be as 1 : 300. The 

 Cactaceae, however, besides exposing 

 a small surface, have other xerophilous 

 characteristics, in their weakly developed 

 intercellular system, their thick cuticle, 

 and so forth ; the Aristolochia, on the 

 other hand, is not only large-leaved, but 

 is also provided with other aids to trans- 

 piration, such as large intercellular sys- 

 tem, thin cuticle, and the like; so that 

 the ratio of their transpiration is, not 

 1 : 300, but 1 : 6,oco. This latter ratio 

 gives a clearer idea of the efficiency of 

 the means for regulating transpiration 

 than any actual description, and it is not 

 at all an extreme case, for although 

 Cactaceae are typical xerophytes, yet 

 Aristolochia is by no means a decided hygrophyte. 



In many hygrophytes, especially those of damp 

 tropical forests, adaptation in the construction of the 

 thin leaf-surfaces is evident. Where the plant is 

 liable to heavy rainfall the leaves have often the 

 long ' dripping point,' by means of which water is 

 soon drained off 2 (Fig. 23). The leaves of plants 

 living in the deep moist shade of the forest, as well 

 as those growing beside brooks where they may 

 be sprayed by the water, frequently have a velvety 

 surface on which the water spreads out by capillarity 

 into an extremely thin layer that evaporates rapidly 

 (Fig. 24) 3 . 



1 This is tropophilous and therefore hygrophilous during the growing season. 

 2 Jungner, op. cit., and especially Stahl, II. 3 Stahl, IV 



C 2 



Fig. 23. Hygrophilous structure. Ficus 

 religiosa. Leaf with dripping point. After 

 Stahl. 



Fig. 24. Hygrophilous 

 structure. Begonia im- 

 perialis. Conical papillae 

 on the upper surface of the 

 leaf. Slightly magnified. 

 After Stahl. 



