lOIO 



A TEXTBOOK OF THEORETICAL BOTANY 



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Fig. 998. — Lithops pseudotruncateUa. Plant showing 

 ocelli as dark spots on the succulent leaves. 



A function of the epidermis which must not be overlooked is that of water 

 storage. The amount stored is not normally large, and it would not suffice 

 to supply for long the quantities which are lost by stomatal transpiration, but 

 under conditions of wilting, when the stomata are closed, the epidermis is 

 the first line of defence against cuticular transpiration, and it can be observed 

 that in wilting leaves the epidermal cells shrink and collapse from water loss 

 before the mesophyll cells are affected. As the radial walls of the epidermal 

 cells are usually thin and are in any case covered with numerous pit areas, it 

 is possible for water to move laterally in the epidermis without much difficulty, 

 and water may in this way be supplied to specially exposed spots on the leaf 

 without diminishing the water content of the photosynthetic cells. A case 

 where this may be important is that of undergrowth plants in tropical forests, 

 subjected to the passage of sun flecks which produce for short periods a 

 marked local heating and drying of the air in contact with the leaf. 



The Stomata. 



Although the stomata are a part of the epidermal structure they deserve 

 special description, not only for their intrinsic importance, but also because 

 they are not physiologically a part of the protective system which is formed 

 by the rest of the epidermis. They are the portals of gaseous exchange between 



