Grossenbacher Radial Growth in Trees. 65 



not been carried too far, the restorative process may go even be- 

 yond the limit attainable under normal conditions. ' ' 



Since Hartig laid especial stress on the difference in the thick- 

 ness of cell walls rather than the size of cells as the essential 

 difference between spring and summer wood his secondary fac- 

 tor, the relative intensity of the transpiration current, would 

 come in for first consideration because it is claimed to regulate 

 the size of cells. It seems possible that the full report prom- 

 ised by Jaccard 129 on the tree-trunk as a shaft of equal water 

 conductance may throw more light on Hartig 's idea. 



The possible relation of enzymes to the formation of "annual" 

 rings: In cases of this kind in which the hypotheses are so 

 numerous and the advocates of each can marshal at least a por- 

 tion of the observed facts in support of their views the truth 

 usually lies somewhere between them, and each conflicting ex- 

 planation will eventually contribute certain fragments to a 

 theory that will account for the known facts. The time for such 

 a theory has not yet come. However, since none of the pro- 

 posed hypotheses gives promise of becoming such an explana- 

 tory theory it may be pardonable to submit yet another with 

 the hope that the viewpoint thus suggested might lead to a new 

 attack on the problem. 



From our present knowledge it seems that to be of any value 

 as a basis for work or a stimulus for the further study of radial 

 growth rings such an hypothesis must, by using all known and 

 some probable but undetermined facts explain how it is that 

 wood cells have a smaller radial diameter in summer than in 

 spring and why vessels are often wholly lacking in the later 

 summer wood. 



It has been shown that an "annual" ring consists essentially 

 of a sheath or ring of wood produced during one more or less 

 continuous radial-growth period and that it is made up of two 

 types of wood which may merge gradually into each other or 

 join at a rather abrupt line. That portion of the ring devel- 

 oped in "spring" or during the early part of a new elongation- 

 growth period has larger cells than that produced in "summer" 

 or after the closing of the first elongation, following the princi- 

 pal dormant season. In the case of trees in temperate zones 

 and many of those in the tropics which produce new leaves near- 



129 1. c. 



5 s. A. 



