THE PLANT BODY 



83 



numerable adaptations in the form of leaves, but the funda- 

 mental structure of all can be reduced to a common plan which 

 may be illustrated by a transverse section of a leaf. (Fig. 47.) 

 The essential features of a leaf consist of upper and lower 

 limiting membranes (epidermis) which are continuous at the 

 edges of the blade, and thus enclose the supporting and con- 

 ducting tissues consisting of vascular bundles (veins), and 



FIG. 47. Cross section of a typical leaf, highly magnified, a, air spaces; b, vein; e, 

 e', upper and lower epidermis; p, palisade layer of chlorenchyma; s, stoma; sp, 

 irregularly arranged 'spongy' chlorenchyma cells. (From Bergen and Davis.) 



the chlorophyll-bearing cells (chlorenchyma) which carry on 

 the work of photosynthesis. 



The walls of the epidermal cells are impervious to water 

 and gases, and therefore the epidermis is perforated with tiny 

 pores (STOMATA) which lead into air spaces among the chlo- 

 renchyma cells. It is estimated that the number of stomata 

 in the epidermis of common leaves averages about 500 per 

 square millimeter, so there is ample provision for the exchange 

 of water vapor, carbon dioxide, and oxygen with the atmos- 

 phere. Each stoma is enclosed by two specialized epidermal 

 cells, termed GUARD CELLS, which regulate the size of the 

 opening according to varying internal and external conditions. 



