42 BRANCHING [CH. 



shoots, either by the displacement of buds or by directive 

 action during elongation. 



Some of these points have been indicated before, but 

 we will now examine them in more detail, and with 

 specific examples. 



Some very striking examples of the departure of an 

 older branch-system or leaf-crown from the normal that 

 would result if all branches persisted, have been collected 

 by Wiesner. 



An Oak-tree 100 years old would have 99 generations 

 or orders of branches if all persisted : as a matter of 

 observation only five or six could be found. A Plane-tree 

 50 years old would have 49 orders, but only seven could 

 be detected. 



Any one can easily make out similar results for him- 

 self if he examines the twigs and branches of any ordinary 

 tree, bearing in mind that the year's growth is defined by 

 the intervals along the axis between any set of bud-scale 

 scars and the next, or the terminal bud. Or, to put the 

 matter in a slightly different form. If a branch of Birch 

 developed two lateral shoots the first year, and in each 

 successive year each of these laterals and its parent-shoot 

 repeated the process, the whole branch-system at the end 

 of ten years should exhibit nine orders of branches tearing 

 altogether 19,683 foliage bearing twigs; but Wiesner was 

 not able to trace more than 238 such twigs on a well- 

 lighted branch of Birch ten years old, and a shaded branch 

 of the same age only showed 182 ; and in both cases, 

 instead of nine orders of branches only five were to be 

 found. 



Apart from the stem and the buds, Wiesner points 

 out that our native trees never show clearly more than 

 eight orders of branches (Hornbeam, Beech, Yew) and 

 usually not more than six orders. 



