BIOLOGY OF HIGHER SEED PLANTS 15 



elm. ? Be able to explain any difference observed in 

 this respect. As the elm branch grows, what becomes 

 of the stronger and the weaker branches ? 

 e. Drawing. Remove the leaves from the branch under 

 observation. Draw the entire branch in outline, 

 indicating clearly the following structures : 



(1) The limits of each season's growth in the length of 



the main axis and the lateral branches. 



(2) The vigorous and the weaker branches of each season. 



(3) The vigorous and the weaker buds in the terminal 



twig of the main axis and one or two lateral 

 branches. 



2. The plan and development of the tree. Study young and 

 mature elm trees in the field, beginning observations 

 with a young elm. 

 a. Younj elms. 



(1) Observe carefully the main trunk or central axis 



of a young elm and the arrangement of the 

 branches. Compare the arrangement of the vigor- 

 ous and the weaker branches of the tree with 

 that of the branch studied above. Can you 

 determine the limits of annual growth on the 

 main trunk, as you can on a branch, by the vigor- 

 ous lateral branches produced each season ? Are 

 the weaker branches being pruned by shading ? 

 Consult Fig. 6 and the text discussion on page 21. 



(2) Do you find anything corresponding to the false 



whorls of branches of the pines and spruces? 

 How does the elm differ from the latter trees 

 in the production of false whorls of vigorous 

 branches ? Did the trunk of the elm below the 

 present branches ever have lateral branches ? If 

 so, why is the trunk smooth now ? 



