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THE MAINTENANCE OF THE INDIVIDUAL 



and externally the phloem tissue. In the spring when this tissue is 

 very active, it forms a soft layer of cells that allows of the easy sepa- 

 ration of bark from wood, a fact well known to any small boy who 

 has made a willow whistle. 



It is not necessary to go into the details of stem structure, except 

 to note that the cambium layer gives rise each year to new layers of 



Cross section of stem of Ricinus communis, a dicotyledon, showing cambium 

 ring. In what area of the diagram does growth take place ? 



tissues, both internally and externally. The inner layers made up 

 of secondary xylem are from the annual rings of a tree. In spring 

 the growth of the tissue is rapid, while in winter it is very slow indeed 

 or stops entirely, thus making the differences in the cross section 

 shown in the figure. As the tree ages, changes may be noticed in the 

 appearance of the older woody area forming the interior of the trunk. 

 This wood becomes darker in color, its chemical composition changes, 

 and it loses its ability to conduct water. It is known as the heart- 

 wood as distinguished from the outer rings of wood called sap-wood. 



