LOSS OF WATER 255 



various modifications and their distribution are very difficult 

 to explain. Most of the stomata are found, in general, on the 

 underside of the leaves. Here they can permit the entrance of 

 the carbon dioxide as well as if they were on the upperside but, 

 protected from the sun, the transpiration will be less. The follow- 

 ing table from Weiss, Eckerson, and others indicates the distribu- 

 tion per square millimeter of the stomata on the leaves of various 



plants : 



Upper Under 



il ant surface surface 



Acer pseudoplatanus (Norway maple) 400 



Betula alba (white birch) 237 



Ficus elastica (rubber plant) 145 



Juglans nigra (black walnut) 461 



Moms alba (white mulberry) 480 



Olea europsea (olive) 625 



Cucurbita pepo (pumpkin) 28 269 



Ly coper sic um esculentum (tomato) 12 130 



Phaseolus vulgaris (bean) 40 281 



A vena sativa (oats) 48 27 



Zea mays (corn) 94 158 



Triticum sativum (wheat) 33 14 



Nymphsea alba (water lily) 460 



Pinus strobus (white pine) 142 



Plants growing in dry localities tend to have a reduced number 

 of stomata (grasses and sedges) or to have the stomata sunken in 

 pits (yucca, pine, and carnation). Other means of checking tran- 

 spiration are by reducing the entire leaf surface. This is seen in 

 plants such as the cactus (Opuntia), paloverde (Parkinsonia) 

 ocotillo (Fouquieria), and in many other desert plants where the 

 leaves have been dropped entirely or in part and a large proportion 

 of the photosynthesis is carried on by the stems. Other plants 

 (cabbage, sugar cane, wheat) prevent transpiration by means of 

 a waxy bloom upon the cuticle, which is thus made impervious to 

 water. Such modifications are commonly associated with an ex- 

 ceptionally thick cuticle as seen in the agave, pine, and yucca. 



The mullein (Verbascum), olive, silver poplar, and buffalo berry 

 (Shepherdia) possess a thick coating of hairs on the under surface 

 which are thought to protect against excessive transpiration, but 

 the work of Sayre (1920) has shed doubt upon their efficacy. The 

 possession of large quantities of water-storage tissue rich in pen- 

 tosans and other water-attracting materials such as is found in 

 the cacti, Agave, Yucca, and many desert plants is of advantage, 

 and there can be no doubt that such " adaptations " have had sur- 



