FOLIAGE LEAVES. 



393 



cells of this layer are narrow and long. This is often beauti- 

 fully shown when comparing 

 leaves of the same plant grown 

 in strong light with those grown 

 in the shade. The compass 

 plant (Lactuca scariola) affords 

 an interesting example. The 

 leaves grown in the light are 

 usually vertical, so that the light 

 reaches both sides. Such leaves 

 often have all of the mesophyll 

 organized into palisade cells (fig. 

 435), while leaves grown in the 

 deep shade may have no palisade 

 cells. 



764. (2) Protective covering. 

 Epidermis and cuticle. The 

 walls of the epidermal cells are 

 much thickened in some plants. 

 Sometimes this thickening occurs 

 in the outer wall, or both walls 

 may be thickened. Variation in 

 this respect as weir as the extent 

 of the thickening occur in dif- 

 ferent plants and are often corre- 

 lated with the extremes of conditions which they serve to meet. 

 The cuticle, a waxy exudation from the thick wall of the epider- 

 mis of many leaves, also serves as a protection against too great 

 loss of water, or against the leaf becoming saturated with water 

 during rains. The cabbage, carnation, etc., have a well-developed 

 cuticle. The effect of the cuticle in shedding water can be nicely 

 shown by spraying v/ater on a cabbage leaf or by immersing it in 

 water. Sunken stomata also retard the loss of water vapor. 



Covers o] hair or scales. In many leaves certain of the cells 

 of the epidermis grow out into the form of hairs or scales of 

 various forms, and they serve a variety of purposes. (Vhen 



ll.S 



Fig. 435- 



Structure of leaf of Lactuca scariola. 

 Upper one grown in sunlight, palisade 

 cells on both sides. Lower one grown 

 in shade, no palisade tissue. 



