PINE LEAF 245 



the inner, or morphologically upper surface of the leaf 

 is flat, while the outer or lower surface is rounded, and 

 the whole leaf is traversed from end to end by two sharp 

 ridges which are slightly rough to the touch. 



Cut transverse sections of a foliage-leaf taken from a 

 stem of the current year. It maybe found convenient 

 to embed in paraffin, or to hold the leaf between pieces 

 of pith, or carrot. Mount some in glycerine, others in 

 chlor-zinc-iodine, and examine with a low power. Note 

 the semilunar form of the section : the flat side is the 

 upper, the convex side the lower. Starting from the 

 periphery observe successively the following tissues : 



1. A single layer of epidermal cells with very 

 thick walls : enlarged cells are to be found at the two 

 corners, and since these cells project slightly they 

 cause the roughness above noted. 



2. A narrow band of thick-walled hypoderma. 



3. A broad band of chlorophyll-containing meso- 

 phyll, with resin-passages. 



4. A bundle-sheath or endodermis, consisting of 

 oval cells. 



5. A broad band of tissue without chlorophyll (the 

 pericycle), which surrounds 



6. Two central vascular bundles. 



Study these several tissues under a high power. 



1. The epidermal cells have their thick walls 

 differentiated into three layers. These may be re- 

 cognized without staining, or better after treatment 

 with chlor-zinc-iodine, as 



i. A thin external cuticle, not very deeply stained : 

 it extends as wedge-like processes between the cells. 



ii. The cuticularized layers, forming a thick band, 



