CHAPTER X. 

 LEAVES, THEIR STRUCTURE AND MODIFICATIONS. 



139. The leaf is an organ of a plant which performs several 

 kinds of very important work. Its structure is remarkably well 

 adapted for these kinds of work and also for its own protection, 

 as well as for the protection of the plant against certain un- 

 favorable conditions of the environment. A study of this struc- 

 ture is necessary to a clear idea of the work of the leaf. 



1. STRUCTURE OF LEAVES. 



140. The epidermis. The epidermis is the outer layer of 

 cells; the upper and lower epidermis covers the upper and lower 

 surfaces of the leaf respectively. The epidermal cells are usually 



Fig. 73- 

 Portion of epidermis of ivy, showing irregular epidermal cells, stoma and guard cells. 



devoid of chlorophyll. The cells are usually thin, and flat in 

 proportion to their extent. Viewed on the surface, as they can 

 be seen by stripping off a portion of the epidermis, the cells in 

 many plants are seen to be very irregular in outline, as in fig. 73. 



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