NO. I GROWTH LAYERS IN TREE BRANCHES — CLOCK ET AL. 39 



exhaustion of the water supply. Springwood is composed of cells which are 

 large, thin-walled, and light-colored, whereas summerwood is composed of cells 

 which are small, thick-walled, and dark-colored. As a rule, springwood merges 

 gradually into the summerwood and the latter terminates abruptly in a sharp 

 outer face. (P. 7.) 



Many growth layers, of course, are annual and "normal" in the 

 sense of possessing simplicity as described. Nonetheless, it was 

 recognized in 1937 that the outer face of the densewood of a "false" 

 (intra-annual) ring might so closely approach the sharpness of a true 

 annual as to give a high degree of uncertainty in identification. 

 "There is nothing to do in doubtful cases except to compare the se- 

 quence with others, to examine the rings throughout more of the 

 trunk, or to discard the specimen" (Clock, 1937, p. 10). It should be 

 our desire to understand the specimens, not discard them. The refer- 

 ence to an examination of more of the trunk undoubtedly presupposes 

 that ( I ) the outer face of a growth layer which approaches an annual 

 in sharpness in one area of the trunk will break down to diffuseness 

 elsewhere, thus revealing its identity as a "false" ring, and that (2) 

 annual sharpness is a quality distinguishable from any sharpness an 

 intra-annual may have. In reference to point (i) we have found "an- 

 nuals" which break down to dififuseness and we have noted intra-an- 

 nuals which, within the limits of our search, maintain their sharpness. 

 Some years of search, in reference to point (2), have failed to reveal 

 any criteria by which sharpness developed at the end of the so-called 

 growing season can be distinguished from that developed by the ces- 

 sation of growth within a season. 



The work upon which this report is based has shown emphatically 

 that the subject of contact surfaces between growth layers is highly 

 complex. As a matter of fact, the subject divides itself into two 

 phases, one having to do with anatomy and description, the other with 

 physiology and cambial activity. The many types of growth layers 

 as marked off by their contacts constitute signposts that act as in- 

 dicators of that activity to which a description, or classification, of 

 growth layers is a necessary preliminary. An understanding of place, 

 time, and rate of cambial activity depends upon a thorough description 

 and classification of growth layers in all their complications. In this 

 matter of classification, contacts play a highly important role. 



Throughout the analyses of the stained microscope sections, four 

 types of contacts were recognized on cross sections, namely, sharp, 

 definite, indefinite, and diffuse. A sharp contact is smooth and pre- 

 sents such a decided contrast between the densewood on the inside and 

 the lightwood on the outside (pi. 2, fig, i : 1941) that no doubt exists 



