NO. I GROWTH LAYERS IN TREE BRANCHES — CLOCK ET AL. 35 



no matter how truthfully the ratio of thicknesses of adjacent growth 

 layers represents their volume ratio where the growth layers are 

 sheaths surrounding the entire plant body. Any single linear measure- 

 ment of a partial growth layer from zero up to the maximum thickness 

 would thus in all probability enter statistical calculations or graphic 

 analyses as either a plus or minus exaggeration. 



The nomenclature used in the classification of growth layers has 

 developed gradually during the course of the work. As has been men- 

 tioned heretofore, almost all growth layers have been studied on a 

 series of cross sections taken along each branch. An attempt has been 

 made to adopt a terminology that will instantly describe the appear- 

 ance of the growth layer on a cross section, at the same time sug- 

 gesting its three-dimensional nature. Consistent with such usage, a 

 growth layer is thick or thin, in radial dimension, rather than wide or 

 narrow (Clock, 1937, p. yz)- 



CONSTITUENTS OF A GROWTH LAYER 

 TERMS 



Our knowledge of furniture, woodwork, and lumber of all kinds 

 has familiarized us with the common pattern of alternating light and 

 dark bands in wood. This same alternation is shown on a log or on 

 any surface cut transversely to the length of the trunk. As is well 

 known, each pair, a light plus a dark band, forms a so-called tree ring, 

 or growth layer. The common idea of a tree ring pictures it as begin- 

 ning abruptly with light-colored wood which passes outward grad- 

 ually into darker material whose outer termination comes abruptly. 

 This succession is repeated in each ring. However, the more one 

 studies not only rings on a cross section but also growth layers 

 along a branch or trunk, the more complex their anatomy becomes. 



So far as the gross constituents of a growth layer are concerned, 

 different points of view may stress time of formation, position in the 

 growth layer, or structure of the tissues. The two parts of a growth 

 layer have been given different names, such as : ( i ) springwood and 

 autumnwood, (2) springwood and summerwood, (3) early wood and 

 latewood, or (4) lightwood and densewood. The first three stress 

 time of formation, a valuable distinction if there is uniformity among 

 trees and if time is distributed correctly. Observation and experi- 

 mentation throw doubt on the strict vaHdity of both suppositions. 

 In the case of springwood and autumnwood no comment need 

 be added, but with springwood and summerwood any objections seem 

 trivial until one wonders whether spring is an astronomic or a climatic 



