THE STEM. 41 



latter are nearly colorless, their very small cavity 

 showing as a dark point at the center. 



3. The inner bark. This is best studied in stems four 



or five years old. It is composed of 



a. Sieve-tubes, narrow elements with light-colored 



walls. 



b. Bast parenchyma, much wider cells frequently con- 



taining chlorophyll. 



c. Bundles of bast fibers similar to those already 



described. 



4. Cambium. In the winter a sharp line of demarcation 



between wood and bark is seen, but in spring there 

 is formed a zone of fresh tissue known as the cam- 

 bium, from the inner cells of which a new layer of 

 wood is produced, and from the outer ones a new 

 layer of bark. See VII below. 



5. The wood. In this observe the following: 



a. Vessels with large openings. 



b. Wood fibers, smaller elements with narrow lumen 



and thick wall. 



c. Wood parenchyma, This is more easily made out 



on longitudinal section. 



d. Medullary rays, extending from the pith outwards 



and continuous with those of the inner bark. 



6. Pith, consisting of very large cells marked by numer- 



ous pits. 



II. Prepare next a number of radial longitudinal sec- 

 tions, mounting as directed above, and study in the same 

 order, comparing them, step by step, with corresponding 

 parts of the transverse sections. 



1. Ascertain whether the cork cells present the same 

 appearance on transverse and longitudinal sec- 



