42 



PLANT RELATIONS. 



but it usually occurs, and does not necessarily imply ex- 

 treme conditions of any kind. However, palisade tissue of 

 unusually narrow and elongated cells, or forming two or 



Fig. 32. Section through a portion of the leaf of the yew (Taxus), showing cuticle 

 (c), epidermis (e), and the upper portion of the palisade cells (p).' 



three layers, indicates exposure to intense light or drouth, 

 and is very characteristic of alpine and desert plants. The 

 accompanying illustration (Fig. 31) shows in a striking 

 way the effect of light intensity upon the structure of the 

 mesophyll, by contrasting leaves of the same plant exposed 

 to the extreme conditions of light and shade. 



The most usual structural adaptations, however, are 

 connected with the epidermis. The outer walls of the epi- 

 dermal cells may become thickened, sometimes excessively 



so ; the other epidermal 

 walls may also become 

 more or less thickened ; 

 or even what seems to 

 be more than one epi- 

 dermal layer is found 

 protecting the meso- 

 phyll. If the outer 



Fig. 33. Section through a portion of the leaf walls of the epidermal 



of carnation showing the heavy cuticle^) jj continue to 

 formed by the outer walls of the epidermal 



cells (ep). Through the cuticle a passageway thicken, the Outer re- 

 leads to the stoma, whose two guard-cells are o-j/yri of the thick wall 

 seen lying between the two epidermal cells ® 



shown in the figure. Below the epidermal loses its structure 



cells some of the palisade cells (pal) are shown a-nA forms the CUticl-6, 



containing chloroplasts, and below the stoma . . . . 



is seen the air chamber into which it opens. W 11 1 C XI IS One 01 tliQ 



