[Chap. XXVII GENERAL REGIONS AND PROCESSES IN STEMS 277 



distance they move through the roots is many times the distance they 

 move through a stem. There are certain species of plants in which photo- 

 synthesis occurs primarily in the stems. There are others in which the 

 absorption of water and inorganic salts occurs primarily in the leaves and 

 stems. The reader may find it both interesting and instructive to add to 

 these examples of similarities among plant organs. 



On the basis of what has been learned so far, it should not be difficult 

 for the reader to formulate intelligent answers to questions that may 

 be asked about stem processes. Why may the girdling of a tree result in 

 its death? Why do people girdle trees? Why may they sometimes fail 

 to obtain the desired results? Why may the leaves soon wilt when the 

 girdle is made deep enough to include a few cylinders of sapwood? 

 If a tree is wounded, will it "bleed to death"? How does healing occur? 

 What is the gravest danger to which a tree is exposed when it is 

 wounded, and how may it be prevented? How do trunks of trees be- 

 come hollow? Why may hollow trees live for many years? If a nail is 

 driven in a tree will it continue to remain the same distance from the 

 base of the tree as the tree increases in height from year to year? Why 

 are there anastomosing longitudinal ridges and crevices in the bark of 

 a tree? What tissue of a tree is used as a source of lumber? Why do plants 

 bearing leaves absorb more water from the soil than they do when 

 leaves are removed? Why may a young tree have a longer growing 

 period during the summer than an older tree of the same species? Why 

 may the new branches of a pollarded tree grow se\'eral feet in length 

 and bear numerous leaves in a single season, while those of an unpol- 

 larded tree may grow but a few inches in length and bear relatively few 

 leaves? Can you add other questions, and answer them? 



REFERENCES 



Chamberlain, C. J. The age and size of plants. Sci. Monthly. 35:481-491. 1932. 



Fry, Walter, and J. R. White. Big Trees. Stanford Univ. Press. 1938. 



Clock, W. S. The" language of the tree rings. Sci. Monthly. 38:501-510. 1934. 



