[Chap. XXVI FORMS AND EXTERNAL FEATURES OF STEMS 



251 



Fig. 85. Pollarding ot trees is followed by the growth ot numerous branches from 

 latent lateral buds in the decapitated stems. Pollarding is practiced where willow 

 shoots are used in making baskets and furniture. From sketch by Van Bosse. 



the buds lower down the stem is known as apical dominance. From pres- 

 ent experimental data it appears to be the eflFect of at least two hormones, 

 one of which is made in the young leaves. If this hormone is artificially 

 applied to the cut ends of decapitated stems it checks the growth of the 

 lateral buds. Apical dominance, through its influence on the relative 

 growth of different parts of the plant, is one of the primary factors in 

 determining tlie form of a plant. 



Buds and stem types. Many trees and shrubs may be recognized at 

 considerable distances by the form of the plant and the type of branch- 

 ing. Such features are often obscured by the effects of environmental 

 factors or by repeated pruning. Plants of the same species may be far 

 from identical in appearance when growing in dense forests and in the 

 open, or along water courses and on dry uplands. The appearance of 

 ornamental shrubs may be changed markedly by the removal of certain 

 dominant buds, thus allowing for the growth of other buds as noted in 

 the paragraph above. 



By comparing a palm, a spruce, and an elm, three types of tree 

 crowns may be distinguished with reference to the mode of branching. 



