BIOLOGY OF HIGHER SEED PLANTS IT 



b. Older elm trees. 



(1) Examine older trees of the American elm with the 

 points iii view which you observed above in 

 young elms. Is the main crown of an old elm 

 tree formed entirely from the so-called false whorls 

 of vigorous branches laid down in the young elm ? 

 Study the mode of growth of the main branches 

 from base to apex. Do they repeat in their growth 

 the history of the tree? Consult Fig. 6 (a-/), 

 and Figs. 12 and 25 of the text. 



c. Summary. Summarize briefly the part played by 



each of the following factors in the development 

 of the elm : 



(1) The spiral body plan and the differences in the buds 



produced each season as regards size and vigor. 



(2) The unequal growth of the buds each spring and 



the difference in the development of the branches 

 produced by them. 



(3) The effects of natural and artificial pruning. 



(4) The annual leaf display. 

 Consult the summary in the text (p. 23). 



SUPPLEMENTARY THEE STUDIES 



Some trees illustrate the above principles relating to tree 

 development more strikingly than the elm. This is particularly 

 true of young Norway maples and the Carolina poplar. Study the 

 development of these and other deciduous trees with reference to 

 the three principles deduced from the study of the pine and the 

 elm. These principles are the body plan or arrangement of leaves, 

 buds, and branches ; the unequal vigor and growth of buds and 

 branches ; the effects of pruning in eliminating all but the most 

 vigorous branches, which then occur in false whorls resembling 

 those of the pine. 



