40 ANATOMY AND PHYSIOLOGY 



the end of one period of growth of xylem (wood) 

 and the beginning of the next? State clearly the 

 differences between the stem at this age and at the 

 age when there is primary growth only. Outside 

 of the wood zone there is the zone of "inner bark." 

 What other term is applied to this tissue? Can 

 you distinguish the cambium layer? Where is its 

 exact location? How do the cells immediately 

 on each side of it differ from the cambium cells? 

 Notice particularly the medullary rays. How 

 numerous are they? Remembering the special 

 properties of cambium, what explanation can you 

 give for the large amount of wood, the complete 

 layer of phloem, and the numerous medullary rays? 

 Do all the rays reach the pith? If not, how far 

 do they reach; and how far do they extend toward the 

 periphery? Are they alike in this respect? How 

 do you account for this condition when compared 

 with the distance the medullary rays extend in 

 primary growth? Can you distinguish separate 

 vascular bundles in this stem? What has caused 

 the difference between this stem and the young 

 Aristolochia stem in this respect? How many differ- 

 ent tissues do you find in the bark region, and what 

 characteristics does each show? 



3. Compare a longitudinal section of this same stem 

 with the cross-section just studied and distinguish 

 each tissue. Notice especially the tracheae. How 

 are they better constructed to give rapid transfer 

 of water than the other parts of the stem ? Note the 

 tracheids (long cells with sharp ends). Are they 

 parenchyma tous or prosenchymatous? Why? 

 Make a list of the various tissues of the stem and, 



