46 PLANT BIOLOGY 



a young stem usually consists of three more or 

 less distinct layers. 



a. With a knife gently scrape off an outer or brown 



bark, and expose a dark green layer known as the 

 green bark. Scrape this until you come to a 

 more or less tough layer known as the fibrous 

 bark or bast (which may be slightly green). 

 Describe each of these barks as to position and 

 color. 



b. Pick into threads the fibrous bark ; in what direc- 



tion of the stem do the fibers run? By break- 

 ing strips of each layer determine which of the 

 three barks is toughest. 



2. Feel of the wood from which the bark has just been 



removed. Describe the substance which covers 

 the wood, after scraping off a little with your 

 thumb nail. This is the cambium or growing 

 layer, which produces the new wood and bark. 

 When the bark is torn off, the cells of this layer 

 are broken and the slimy protoplasm oozes out. 



3. By means of a penknife or pin dig into the wood and 



also into the pith at the center of the stem. 

 Compare the wood and the pith as to relative 

 position and hardness. 



4. By the aid of compasses make a diagram, at least three 



inches in diameter, of the cross section of a 

 woody stem to show the relative thickness of 

 the various layers. (These layers might well be 

 represented by different colors.) Label brown 

 bark, green bark, fibrous bark or bast, cambium 

 layer, wood, pith. 



B. The structure of an older stem. (Optional.) 



Cut some cross sections of stems several years old. (Ad- 

 mirable material can be obtained by sawing into pieces 

 about two inches long white oak sticks three to four 

 inches in diameter.) Each piece should then be split 

 into halves and each surface planed and sandpapered. 

 These pieces are valuable as permanent preparations. 



