PINE LEAF 143 



THE LEAF. 



Examine the bifoliar spur of Pinus sylvestris as a whole : it 

 consists of a very short axis, at the base of which are borne mem- 

 branous sheathing scales, and at the apex two long needle- 

 shaped foliage-leaves : in other species of Pinus the number 

 may be larger. Note that 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 marginal ridges which are slightly rough to the 

 touch. 



Cut transverse sections of a foliage-leaf taken from a stem of 

 the current year. It may be 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 mesophyll, 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. 



I. The epidermal cells have their thick walls differentiated 

 into three layers. These may be recognized without staining, 

 or better after treatment with chlor-zinc-iodine, as 



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

 tends as wedge-like processes between the cells. 



