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PLANT PHYSIOLOGY AND ECOLOGY 



or above them, which serve to reduce the size of the opening. 

 In some cases, moreover, the size of the pore formed by the two 

 guard-cells is permanently reduced. In a few plants the effect 

 of intense drouth is almost completely prevented by closing the 

 pore by means of a waxy excretion. 



170. Changes in the chlorenchym. The rapidity with which 

 water escapes from the tissue of the leaf is largely determined by 

 the size and number of the air passages. Water-laden air reaches 

 the stomata most easily when the air spaces are large and con- 

 tinuous, and least readily when they are small and scattered. 

 Consequently, leaves exposed to the danger of excessive water 



Fig. 47. Leaf of a plains species, Bahia dissecta, in which the chlorenchym 

 consists entirely of palisade tissue. 



loss usually have the size of the air spaces reduced, and especially 

 the size of the passages that connect them with the pores of the 

 stomata. An increase of palisade tissue reduces many of the air 



