Grossenbachsr — 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 



••Metzger, A. Der "Wind als massgebender Faktor fur das Wach- 

 sthum der Baume. Miindener Forst. Hefts. 3:35-86. 1893. 



