PINE STEM 233 



are separated from one another laterally by bands of 

 parenchyma ; that their form is approximately wedge- 

 shaped ; and that the tissues of which they are composed 

 may be distinguished as 



i. Xylem, nearer the centre of the stem, the com- 

 ponents of which have thick, dark-looking, lignified 

 walls (yellow with chlor-zinc-iodine). These first-formed 

 xylem-elements, since they differ from those formed 

 later, are distinguished as protoxylem. 



ii. Phloem, nearer the periphery, with bright-looking 

 cellulose walls (blue with chlor-zinc-iodine). 



The more minute study of these tissues must be 

 deferred for the present. 



Outside the ring of vascular bundles is 



3. The cortical tissue, a mass of cells similar in 

 structure to the pith. In this occur large intercellular 

 spaces, which are resin-passages. Since the periphery 

 of the section of the axis of the bud is complicated by 

 great irregularity of outline, the study of the outer 

 tissues will be better carried out in the older stem. 



II. Cut transverse sections of the stem of the current 

 year : mount some in glycerine, others in chlor-zinc- 

 iodine : the sections have a wavy outline, the inden- 

 tations corresponding to the grooves above observed 

 externally. Starting from the periphery of the section, 

 note the following tissues : 



1. Epidermis, a single layer of cells, following the 

 wavy outline of the section: the walls, especially the 

 outer, are much thickened : externally there is a 

 cuticle. 



2. Cortical tissue, consisting of cells with rather 

 thick cellulose walls (blue with chlor-zinc-iodine), and 



