THE FACTORS 



[Part I 



do not involve any diminution of the transpiring surface. Their internal 

 structure, like their external form, is specially adapted for promoting the 

 exit of water. 



Protective means against the loss of water are not, however, as a rule 

 completely wanting. The Hymenophylleae of moist evergreen forests 

 dry up very quickly in dry air, and are therefore always dependent on an 

 atmosphere that is constantly very damp. This is also true, if to a less 

 degree, of other herbaceous plants of similar habitats. Hygrophilous 

 trees, on the contrary, are at times exposed to a less humid atmosphere, 



and are partially exposed to direct 

 insolation, by which, even in a satu- 

 rated atmosphere, their transpiration 

 is materially accelerated. Hence 

 many hygrophilous woody plants, 

 especially in the tropics 1 , possess 

 distinct though weakly developed 

 protective devices against the loss 

 of water, apparently similar to those 

 which are strongly developed in xe- 

 rophytes, such as an epidermis rich 

 in water or a thin aqueous tissue, 

 and in leaves exposed to sunlight 

 a well-developed cuticle. 



By such protective means the pali- 

 sade-cells are guarded against ex- 

 cessive loss of water during the hot 

 midday hours, the stomata are also 

 closed during the same period. The 

 urgent necessity for such a temporary 

 decrease in the transpiration is proved 

 by the drooping of the foliage of 

 many tropical trees and shrubs under 

 the midday sun. At other hours 

 of the day, or when the sky is clouded, transpiration through the stomata 

 is quite unrestricted 2 , and under weaker illumination far exceeds that 

 through the cuticle. Danger to hygrophytes from too much transpiration, 

 if it exists at all, is limited to a few hours in the day. and is often non- 

 existent for weeks ; it may, at the worst, cause the foliage to wilt, but 

 cannot cause death from desiccation. Their chief danger is that of 

 stagnation of the transpiration-current, and the existence of this danger 

 is primarily apparent in the structure of hygrophytes. 



Fig. 22. Ulex europaeus. a In ordinary 

 air. b In saturated air. After Lothelier. 



1 Haberlandt, I. 



Ibid. 



