THE NUTRITIVE ORGANS OF PLANTS 



51 



tured food substances, and of wood cells that help to strengthen the 

 bundle. 



64. Changes in stems during their growth. In our dis- 

 cussion thus far, we have considered the adaptations of stems 

 for exposing leaves to the light and for transmitting food 

 materials to and from the leaves. But the stem has other 

 important functions which we are now to consider. In a 

 young twig, before the brown bark thickens and shuts out 

 the light, the green bark, on account of the presence of chlo- 

 rophyll, is enabled to carry on the manufacture of carbohy- 

 drates. In a very young, stem the surface is covered by thin 

 epidermis which helps to prevent 

 the undue escape of moisture. In 

 this layer are tiny openings that 

 allow the inward and outward 

 passage of gases that occur in 

 breathing and food manufacture. 

 Later this epidermis is replaced 

 by the outer or brown bark, which 

 serves as a means of protection 

 against unfavorable weather con- 

 ditions and insects. In this brown 

 bark the tiny openings referred 

 to above are developed into large 

 openings known as lenticels which 

 carry on the same functions. Ii> an old tree the outer bark 

 becomes very thick and corky and the green layer dis- 

 appears entirely. 



The growth of the tree in thickness, as already stated, is 

 due to the activity of a layer of cells between the wood and the 

 fibrous bark. This is the cambium layer (Fig. 15). In early 

 spring the cambium cells by rapid growth and division form 

 on their innermost surface a new layer of wood (which appears 



FIG. 17. Cross section of a 

 tree trunk showing bark, 

 wood (with its annual rings 

 and medullary rays), and pith 

 at center. (Courtesy of 

 New York Botanical Garden.) 



