THE CASTOR BEAN 



111 



narily a year's growth, the age of the plant may be determined 

 by counting the number of rings. Such rings are rarely visible 

 in the bark, although the bark also increases in thickness by 

 layers added to its inner side. 



From this description, it is evident that the growing part of 

 the stem is the cambium layer and that the stem of the plant 

 is capable of continuing its life only as long as this cambium 

 layer is intact. What is known as girdling a tree consists in 

 cutting a ring through the bark around the tree in such a way 

 as to destroy entirely the bark and the cambium layer; this 

 effectually kills the tree because the cambium layer is destroyed, 

 and unless there is a connection of living cambium between 

 the roots and the leaves, the life of the plant cannot be main- 

 tained. It is also evident why the bark may be stripped 

 away from the wood 

 of the tree so readily. 

 The inner edge of the 

 bark comes next to the 

 cambium; the cambium 

 cells are thin- walled, 

 full of soft protoplasm 

 and easily broken, and 

 hence the bark is easily 

 separated from the rest 

 of the tree at this point. 



Medullary Rays. - 

 The cells in the vascular 

 bundle extend up and 

 down the stem. There 

 are, however, other 

 cells that run horizon- 

 tally, extending from 

 the center to the outer 

 edge. These form what are called medullary rays (Lat. medulla 

 = marrow); see Fig. 51. They probably serve for the trans- 



$ ph 



FlG. 51. A PARTLY PERSPECTIVE VIEW, 

 SHOWING THE RELATION OF THE PARTS 

 IN THE STEM OF AN OAK 



c, the cambium layer; 

 7, medullary rays; 

 ph, phloem; 



s, stereome cells; 

 x, xylem. 



