[Chap. VIII THE TISSUE SYSTEM OF LEAVES 71 



to mature leaf may occur at a rather uniform rate during ten days to 

 two weeks. In large-leaved plants, such as tobacco, the leaves grow for 

 a month or more before becoming mature. 



Leaf tissues. When we look at the exterior of a leaf we find the upper 

 and lower surfaces composed of angular or interlocking cells of the epi- 

 dermis ( Fig. 34 ) . The outer cell wall contains a deposit of cutin, a fat- 



Upper epidermis 



■ Palisade layers 



Intercellular 

 ^ , y^ 5pace 







Lower 

 epidermis 



Fig. 34. Tissues of a leaf of the common periwinkle {Vinca). Courtesy of World 



Book Co. 



like substance. This cutinized layer is often called the cuticle. It may be 

 very thin, or it may be so thick that the outer wall is the most conspicu- 

 ous part of the epidemial cell. 



In the lower epidennis, and sometimes among the upper epidermal 

 cells, paired specialized guard cells partially separate and form a pore 

 or stomate.- When open, the stomates are passages connecting the air 

 within the intercellular spaces of the leaf with the external atmosphere. 



Between the upper and lower epidennis there are several layers of 

 cells constituting the mesophyll. The upper layers usually have elongated 



- Some authors prefer stoma (Greek = mouth; pi. stomata); in either case the adjective 

 is stomctal. 



