i2 



PLANT STUDIES 



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 



'"y^ 



Fig. 32. Section through a jicjrtion of the leaf of the ye\S' (Tunis), showing cuticle 

 (ci, epidermis (t), ami the u]i])er portifin of the palisade cells ( p). 



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 efEect of 

 light intensity upon the structure of the mesophyll, by 

 contrasting leaves of the same jilant exposed to the extreme 

 conditions of light and shade. 



The most usual structurtil adaptations, however, are 

 connected with the ejiidermis. 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 

 1)0 more than one epi- 

 dermal layer is found 

 protecting the meso- 

 phyll. If the outer 

 walls of the eiiidermal 

 cells Continue to 

 thicken, the outer re- 

 gion of the thick wtill 

 loses its structure 

 and forms the cuticle, 

 which is one of the 



Fig. 33. Section through a jioition of the leaf of 

 carnation, showing the heavy cuticle (<■?/) 

 formed by the outer wails {if the cpidernral 

 cells {€})). Through the cuticle a passageway 

 leads to the stoma, \vhose two guard-cells are 

 seen lying hetween the two cpidernuil cells 

 shown in the figure. IJelow the epidermal 

 cells some of the palisade cells (pal) are shown 

 containing chloroplasts, and below tlic stoma 

 is seen the air chamber into which it opens. 



