



CHAPTER XII 

 MINUTE STRUCTURE OF LEAVES; FUNCTIONS OF LEAVES* 



120. Outline of leaf structure. Most foliage leaves of seed 

 plants contain a rather complicated system of nbro-vascular 

 bundles forming the veins (Sec. 105), which, taken together, con- 

 stitute a framework by which the leaf is supported and strength- 

 ened. Over and around these veins lies a mass of green pulpy 

 material, the spongy parenchyma. . The whole leaf is covered by 

 an epidermis. Frequently, especially in soft and rather thick 

 leaves, such as those of the garden live-forever, the epidermis 

 can be readily peeled off as a thin, transparent skin. 



The epidermis and the spongy parenchyma decay far more 

 readily than the woody framework, and so skeleton leaves may 

 often be found on the ground in the spring, showing plainly the 

 arrangement of the veins of the leaf. 



121. Details of a leaf section. .The relative positions and 

 the detailed structure of the parts mentioned in Sec. 120 are 

 best understood by reference to the magnified cross section of a 

 typical foliage leaf. 



In the ordinary leaf (Fig. 112) a section shows at the upper 

 surface a layer of transparent cells of the epidermis e, ' Beneath 

 this lies a layer of elongated cells p, of a green color, standing 

 at right angles to the epidermis. These are called palisade cells, 

 from a fancied resemblance of their shape and relative position 

 to palisades. Under this layer the leaf interior is filled with an 

 irregularly grouped mass of green cells known as the spongy 

 parenchyma sp, throughout which occur numerous air spaces a, 



* To THE INSTRUCTOR : As the present chapter takes up its topics in con- 

 siderable detail, it is suggested that it may be found expedient, if time is 

 limited, to omit Sees. 129, 130, 132, 134, 139 (table), 145-147. 



102 



