THE FLOWERING PLANT — STEMS 351 



decided variation in thickness ? Does the bark 

 completely cover the stem? From its charac- 

 ters would you judge it to be a living tissue ? 

 Hold a piece of bark up to the light to see if 

 the bark is transparent. Examine bark from 

 different parts of the stem with regard to this 

 point. What do you consider to be the func- 

 tion of this layer ? 



3. The green bark. — How does it compare in 



thickness and texture with the outer bark? 

 To what is its color probably due ? Of what 

 use may this layer be in this part of the 

 plant ? 



4. The bast. — What is its color? How does it 



compare in thickness with the other layers ? In 

 what direction do its fibres run ? Do you find 

 indications of more than one layer ? Of what 

 use is the bast ? 



5. The wood. — How does it compare in color, text- 



ure, and thickness with the other layers ? Ex- 

 amine under a lens to see the annual rings 

 and the medullary rays. How many rings 

 do you find ? Are all of the same width ? Why? 

 How much does the stem increase in diameter 

 in one year? Make cross- sections of other in- 

 ternodes of the same stem, and note the num- 

 ber of rings found in each. Is it the same for 

 all ? How do you account for the results ? In 

 what direction do the medullary rays run? 

 With what parts do they connect? Of what 

 use to the stem is the wood ? Note how easily 

 the bast separates from the wood. 



6. The pith. — How much of the cross-section does 



it occupy ? What is its texture ? Does the pith 



