INTRODUCTION' 



47 



the cuticle is highly variable, in some organs being a hardly per- 

 ceptible "film" while in other instances (e.g., fruits and certain 

 types of leaves) it is extremely prominent (Eames and Mac- 

 Daniels p. 37, Fig. 24). The imtUs of epidermal cells vary in 

 their structure and chemical composition. Typically, the outer 

 tangential vxill directly beneath the cuticle, is the most heavily 

 thickened of all the walls and the irmer-tmigential wall the thin- 

 nest. Often the radial tmill tapers in thickness towards the inner 

 tangential wall. Simple pits are common in the radial and inner 

 walls of epidermal cells. According to Ilaberlandt (p. 102) the 

 innermost zone of the outer wall usually consists of "unaltered 

 cellulose" and is followed externally by layers of wall substances 

 which contain varying amounts of cutin. The recent work of 

 Anderson (1934), however, has shown that in the leaf of CAivia 

 nobilis, the thick outer wall of the epidermal cells shows "two 

 distinct zones of cutinization." The outermost zone is devoid 

 of cellulose or pectin while "the inner zone of cutinized wall 

 consists of a series of cellulose lamellae separated by layers of 

 pectic material, both of which are impregnated with cutin. The 

 inner cutinized zone may be in direct contact with the protoplasm 

 of the cell or may be separated from the protoplasm by a second 

 zone of cellulose and pectic materials." It is clear from this 

 Avork that a thorough study of the process of cutinization in epi- 

 dermal walls of various plants is urgently needed. 



The continuity of the epidermis, especially of foliage leaves 

 and young stems, is interrupted by minute openings or pores 

 which are termed stomata.. Each stoma represents an intercellu- 

 lar space between a pair of highly specialized epidermal cells 

 known as guard cells. As seen in surface view, guard cells are 

 very frequently crescent-shaped with their concave surfaces 

 adjacent to the slit-like pore. In contrast to ordinary epidermal 

 cells, the walls of guard cells are uneven in thickness, often with 

 ridge- or flange-like extensions at the edges of the pore. Further- 

 more, guard cells usually contain prominent chloroplasts. Since 

 stomata play such an important role in the processes of respira- 

 tion, photosynthesis and tranpiration, much attention has been 

 devoted to the "mechanism" by which the stomata are "opened" 

 and "closed." In general, changes in the width of the stoma 



