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’s 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 Bachs and de Yries was made by Detlefsen 40 in ex¬ 
plaining excentric 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 
28 Kny, L. tiber das Dickenwachstum des Holzkorpers der Wurzeln 
in seiner Beziehung zur Lothlinie. Ber. Dent. Bot. Ges. 26:19-50. 1907. 
40 Detlefsen, E. Versuche einer mechanischen Erklarung des ex- 
centrisehen Diekenwachsthums verholzter Aschen und Wurzeln. 
Arbeit. Bot. Inst. Wurzburg. 2:670-88. 1882. 
