Grossenbacher—Radial Growth in Trees. 
35 
mechanical needs in supporting the top in its environment. The 
general form of trunks was found to conform more or less com¬ 
pletely with shafts constructed to be of equal endurance through¬ 
out and capable of supporting a given load (top) and wind-pres¬ 
sure. It is said that owing to this fact a tree trunk grown in the 
open and therefore bearing branches nearly to the ground is 
thicker at the base than one grown in a forest and crowded by 
other trees. 
Some years later Metzger 69 published some results and obser¬ 
vations from which the striking conclusion is drawn that light, 
warmth, moisture and food enable a tree to grow but that the 
wind determines how it shall grow. He points out the self-evi¬ 
dent but none the less interesting fact that a tree-trunk must not 
only carry its own weight and that of the branched top but also 
resist the wind action as it shifts the center of gravity while 
swaying to and fro. The tree stems are said to be the pillars of 
the forest and in order that the forest exist they must be both 
rigid and at the same time elastic enough to withstand strong 
winds. This is illustrated by him by imagining a wooden shaft 
firmly fixed in a horizontal position at one end and weighted at 
the other, thus resulting in the greatest strain at the place of at¬ 
tachment. If such a shaft is to be equally liable to break at any 
point of its entire length its cross sectional area must decrease 
from the point of support to the application of the weight or 
force in accordance with the physical laws involved, and the most 
economical use of the material of the shaft would require such 
a construction. By making numerous measurements and calcu¬ 
lations it was found that the proportional thickness and form of 
tree-trunks below the branch-bearing tops was practically that 
required of the shaft described above, except that most of them 
are enlarged at the base or root-crown beyond the hypothetical 
requirements. It is noted that tap-rooted trees in deep soil are 
devoid of the excessive basal enlargement, and it is therefore 
thought that the enlargement is only a result of developing an 
adequate root anchorage for the tree. That portion of the stem 
in the branching top was also found to conform to such a shaft. 
In case of horizontal branches it is held that their own weight 
overbalances wind action as a formative factor, while in upright 
69 Metzger, A. Der Wind als massgebender Faktor fur das Wach- 
sthum der Baume. Miindener Forst. Hefts. 3:35-86. 1893. 
