PLANT RELATIONS. 



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

 treme conditions of any kind. However, if the cells of the 

 palisade tissue are unusually narrow and elongated, 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 Q/). 



form two or three layers, we might infer the probability of 

 exposure to intense light or drought. 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 



_ Cfi wa U g ma J a l so 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 

 walls of the epidermal 

 cells continue to 

 thicken, the outer re- 

 gion of the thick wall 

 loses its structure 

 and forms the cuticle, 

 which is one of the 



FIG. 33. Section through a, portion of the leaf of 

 carnation, showing the heavy cuticle (cu) 

 formed by the outer walls cf the epidermal 

 cells (ep). Through the cuticle a passageway 

 leads to the stoma, whose two guard-cells are 

 seen lying between the two epidermal cells 

 shown in the figure. Below the epidermal 

 cells some of the palisade cells (pal) are shown 

 containing chloroplasts, and below the stoma 

 is seen the air chamber into which it opens, 



