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THE AMERICAN NATURALIST [Vol. XL 



versicolor are suited by the heavy outer epidermal wall, which 

 is cuticularized, as well as by the stomata of peculiar structure, 

 to resist rapid loss of water. This is the type of structure that is 

 to be found during the periods of drought. The embryonic por- 

 tions of these cacti, and the evanescent organs, in which are 

 included the leaves of Opuntia, are well adapted to promote tran- 

 spiration. This is accomplished in the embryonic tissues by a 

 thin epidermal wall and by the undifferentiated portions of the 

 outer part of the cortex by which a rapid transfer of water is possi- 

 ble. The substomal tube functions also throughout its entire 

 length as the substomal chamber. In the leaves of Opuntia not 

 only is the epidermal wall delicate, but the outer cortex is never 

 differentiated into sclerenchyma and chlorenchyma and there is 

 no substomal canal. Such is the structure of the tissues at the 

 times when the rate of transpiration is most active. 



5. The leaves of Opuntia versicolor play an important role 

 in transpiration. In one instance with somewhat less than one 

 fourth the entire transpiring surface the leaves transpired nearly 

 one half the whole amount. In another instance about 45 per- 

 cent of the entire transpiring surface was foliar and the leaves 

 transpired about 55 percent of the total amount. 



Desert . Botanical Laboratory 



