CHAPTER XI. 

 LEAVES, THEIR STRUCTURE AND MODIFICATIONS. 



141. Structure and uses. 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 unfavorable conditions of the environment. 

 A study of this structure is necessary to a clear idea of the work 

 of the leaf. 



i. MICROSCOPIC STRUCTURE OF LEAVES. 



142. 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. 74- 



rig. 74- 



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



devoid of chlorophyll (paragraph 147). The cells are usually 

 thin, and flat in proportion to their extent. Viewed on the 

 surface, as they can be seen with the microscope after strip- 

 ping off a portion of the epidermis, the epidermal cells in many 



