REVIEWS . 



Tropismen und excentrischcs Dickenwachstum der Bdmne. By Ar- 

 nold Engler. Beer & Company, Zurich, 191 8. Pp. 106 2°. Price, 

 frcs. 10. 



This prize essay by the well-known director of the Swiss Forest 

 Experiment Station is the result of eighteen years' systematic observa- 

 tions, tabulated measurements, and experiments to account for the 

 eccentric growth and form of trees under various conditions. 



While considerable theoretical interest attaches to this problem, it is 

 not without practical interest, especially in the practice of thinnings. 



In the introductory five pages the author briefly rehearses what other 

 authorities have produced regarding geotropic, heliotropic, and other 

 influences on the uneven development of diameter growth and form. 



His own observations show that twigs, branches, and even stout 

 stems of all broadleaf and coniferous species, are capable of geotropic 

 curvatures, so that a more than 5-inch stem, brought out of vertical 

 position, can erect itself and re-establish verticality. Reactions of such 

 nature to heliotropic stimuli are also observed in broadleaf branches 

 and stems, but not in coniferous several years old. In the former 

 species, both geotropic and heliotropic curvatures are very frequent, 

 and account for the form of their crowns. This reaction of older 

 woody parts is, however, induced by or in relation to the young shoots. 

 It is due to this ability that broadleaf trees bend and grow into the 

 openings made by thinnings, while spruce and fir do not respond in the 

 same way. 



To the peculiarities of form in broadleaf trees on steep slopes a spe- 

 cial chapter is devoted. On such slopes, in even-aged, close stands, as 

 a rule, will be found a strong curvature at the base, convex toward the 

 valley ; then the branchless part is found straight, but leaning valley- 

 wards, and the top curving from the base of crown, concave toward the 

 valley. This form is also assumed by pine and larch ; only in fir and 

 spruce on steep slopes vertical position, as taught in text-books, is found. 

 The influence of different methods of thinning on form is discussed. 



Gravity and light are the causes of form, which on steep slopes do 

 not work in the same direction, the light being more one-sided, and 

 curtailed on the upper side. Since the trees on such slopes arc leaning 



71") 



