Grossenbacher — Radial Growth in Trees. 19 



ments by Lutz show in addition that food distribution in stems 

 is related to the source and amount of elaborated food descend- 

 ing from the leaves although such factors do not seem to pre- 

 vent the eventual regional distribution so strongly brought out 

 by the observations of Fabricius, except in cases where the sup- 

 ply is very limited and apparently all used up or deposited on 

 its way down the tree. At any rate, in such instances too little 

 reaches the lower part of the trunk and roots to permit the oc- 

 currence of radial growth in those regions. Some recent defolia- 

 tion experiments by Kiihns 33 show that the radial growth occurr- 

 ing after defoliation usually does not extend to the base of the 

 stem and, therefore, results in an incomplete double ring. In 

 this case as in those cited by Hartig, Rubner, etc., the conclusion 

 seems warranted that radial growth was omitted on the lower 

 part of the trunk and roots chiefly because the downward stream 

 of elaborated food is used up before reaching that part of the 

 trunk. "When the growth of excentric roots and an irregular 

 distribution of radial growth at any given circumference of a 

 tree-trunk, as noted by Lutz, are considered in relation to the 

 occurrence of reserve food, the problem becomes more complex. 

 Such cases make it necessary either to assume that elaborated 

 food is thus irregularly distributed in a tree or else that other 

 factors are involved .in the distribution of radial growth. 

 Fabricius found that food is stored in a larger number of rings 

 on the thicker side of an eccentric root, but that does not neces- 

 sarily mean that the oldest starch-bearing rings on that side are 

 any older than the oldest starch-bearing ones on the thinner side 

 since rings are often entirely omitted on the narrower side. It 

 is at least possible that radial growth begins in spring in that 

 portion of a tree in which the greatest amount of food is stored 

 and in view of the fairly well established fact that growth con- 

 tinues longest in fall in such regions of maximum food content 

 this possibility is somewhat emphasized. Perhaps it might be 

 of interest first to consider the causes of excentric growth as far 

 as they have been determined before taking up the factors which 

 have been advanced by several authors as the cause not only of 

 the distribution of reserve foods but of the general form of tree 

 trunks. 



!3 Kiihns, R. Die Verdoppelung des Jahresringes durch kiinstliche 

 Entlaubung. Biblio. Bot. 70:1-53. 1910. 



