42 



THE ECOLOGY OF LEAVES 



On the other hand if we examine the leaves of plants growing in 

 progressively drier places we shall find that there is a progressive 

 decrease in the amount of spongy parenchyma and a progressive 

 increase in the amount of palisade tissue until in the case of plants 



Fig. 15.— Sectional and surface views of an ordinary leaf. A, upper epider- 

 mis; B, palisade layer; C, spongy layer; D, lower epidermis; E, stoma; F, air 

 space; G, vein. (From Sinnott's Botany— Principles and Problems, New York, 

 McGraw-Hill Book Company, Inc.) 



growing in very dry places there is no spongy parenchyma at all and 

 the chlorenchyma may consist entirely of palisade tissue (Fig. 17). 

 There are, of course, many exceptions to these general conditions, 



Fig. 16.— Section of a leaf of a water plant, pondweed {Potamogeton). UE, 

 upper epidermis; SP, spongy tissue; LE, lower epidermis. 



especially in dry regions where very great variations in leaf structure 

 occur. 



There are three general types of these exceptions that are suffi- 

 ciently common to be worthy of mention here, although this by no 



