Grossenbacher—Radial Growth in Trees. 
31 
with the occurrence of the upper lateral roots. In trees like the 
elms, ironwoods, and oaks the excessive thickening in the upper 
angle primary roots make with trunks are often exaggerated into 
buttress-like enlargements which are continued as ridge-like pro¬ 
longations extending some distance up the trunks. According 
to Detlefsen 58 the excessive radial growth in the upper angle of 
lateral roots and in the lower angle which large branches make 
with the trunks is chiefly due to a continued decrease of the bark 
pressure at these places which results from radial growth. This 
hypothetical explanation, however, requires an experimental 
basis. The fact that the bark at these places is often cleft or 
ruptured rather shows that radial bark pressure, at least, occurs 
there. The pressure exerted against the bark by the growing 
wood is not only sufficient to bring about tension at the root and 
branch ridges but tension of sufficient magnitude to rupture the 
bark in many instances. The experiments by Vochting 59 in 
which the distal tips were cut from Helianthus and other plants 
with the result that the stems became somewhat fleshy and in 
some cases rib-like thickenings developed over the leaf traces and 
ran some distance down the stem, can scarcely be said to apply 
owing to the fact that in Vnch ting’s experiments the excessive 
thickening was chiefly due to increase in the pith and cortical 
parenchyma instead of radial growth of the stele. 
It has been suggested or inferred by some of the above as well 
as by other writers that greater cambial activity occurs in the 
upper angle of roots at their origin from the stump than takes 
place in the lower angle, because the downward current of meta¬ 
bolized food is checked and accumulates more or less in the up¬ 
per angle. The lower angle of the root is said to be more indi¬ 
rectly and, therefore, more sparsely supplied with food and for 
that reason one sided radial growth results. An additional 
factor, which contributes to this excentricity, is doubtless the 
pressure of the tree’s weight on the cambium of the underside 
and another may be the reduced longitudinal bark tension sug¬ 
gested by Detlefsen. Even in case of a tree with a deeply pene¬ 
trating tap root a very marked radial increase on the lower side 
of large primary laterals would tend to elevate the entire tree, 
and a tree without a tap root must be carried chiefly by the large 
68 1. c. 
69 1. c. 
