Grossenbacher — Radial Growth in Trees. 23 



clambering up vertical walls were found to have both the wood 

 and phloem portions of the bundles thicker and of larger cells on 

 the wall than on the free side of ascending branches which is as- 

 sumed to have become inherited dorsiventrality. 



Kny's study of the roots of both hyponastic and epinastic 

 species showed that no regularity occurs in the excentricity of 

 radial growth and it was thought that local pressure relations 

 may determine the excentricity in roots. The lateral roots were 

 cut from small seedlings of Tilia, Picea and Gleditschia and, 

 after they had begun to develop new roots, they were placed in 

 darkened Knop '& solution and allowed to grow. No excentricity 

 resulted except in some cases where the upper radius was greater 

 at the origin of the root from the axis. An examination of hori- 

 zontal roots which had been exposed for years, showed that their 

 excentricity is the same as that of the branches of the same tree. 

 In a more' recent paper he 39 came to practically the same con- 

 clusions and maintained that the same factors which induce ex- 

 centric growth in aerial structures are in the main responsible 

 for their occurrence in roots. The atmospheric environment was 

 thought somehow to be the causal agent. 



A new and rather striking application of the bark-pressure 

 hypothesis of Sachs and de Vries was made by Detlef sen 40 in ex- 

 plaining exeentric radial growth. He pointed out the obvious 

 fact that on the concave side of a curved stem radial growth must 

 necessarily decrease while on the convex side it increases bark 

 pressure chiefly because of the effect such growth has upon lon- 

 gitudinal tension of the bark. Owing to the presence of the 

 hard-bast fibers in the bark the reduction of the pressure on the 

 cambium becomes effective some distance on both sides of the 

 curve. The bark was usually found to be considerably thicker 

 on the side of a stem having the greater radius and it was fre- 

 quently wrinkled or at least more rugged. He held, therefore, 

 that the excessive thickening in the upper angles of large lateral 

 roots and in the lower angle of branches is due to the reduced 

 bark pressure at those places following radial growth, and that 

 the ridges extending from such roots up the trunks are secondary 



'"Kny, L. tiber das Dickenwachstum des Holzkorpers der "Wurzeln 

 in seiner Beziehung zur Lothlinie. Ber. Deut. Bot. Ges. 26:19-50. 1907. 



40 Detlefsen, E. Versuche einer mechanischen Erklarung des ex- 

 centrischen Dickenwachsthums verbolzter Aschen und Wurzeln. 

 Arbeit. Bot. Inst. Wiirzburg. 2:670-88. 1882. 



