Grosseribacher Radial Groivth 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 Yochting'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 



8 1. c. 

 " 1. c. 



