CHAP, iv STEMS 23 



The stem of a tree is said to be either arboreus 



/ * f / 



or arborescent. What is the difference? ju/^ t j 



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XII. Examine the surface of a branch of Walnut or But- 

 ternut which has been cut across with a sharp knife (or 

 razor), and with the aid of the lens notice : 



1 . The central pith. 



2. The ring (or rings notice the number in different 



twigs) of wood (xylem) about the pith. 



3. The fine white lines radiating out from the pith through 



the wood. These are the medullary ra\s. 



4. Outside the rings of wood, a narrow whitish ring, the 



inner part of which is the cambium, the outer part 

 the soft bast. 



5. Outside of this a ring of dots, the hard bast. 



6. Then a fairly wide, yellowish green ring, the cortex, and 



finally 



7. A narrow brown ring, the corky bark. 



8. Make a sketch, representing and labelling the parts. 



XIII. i. Is the stem just studied, exogenous or endoge- 

 nous ? Why ? 



2. How old was the branch you were studying? 



3. How may we tell the age of a tree? 



4. Can you name any tree or trees possessing the other 



kind of stem? 



5. The trunk and branches of exogenous trees increase in 



girth through the activity of the cambium ring which 

 forms new wood or xylem on the inner side and new 

 bast or phloem on the outer side. Most endogenous 

 stems, having no cambium ring, do not increase very 

 much in girth. 



