NO. I GROWTH LAYERS IN TREE BRANCHES — CLOCK ET AL, 53 



middle one partly sharp, complete, and entire, and the outer one sharp, 

 complete, and entire. Over an arc of 15° the outer contact of the 

 middle grov/th layer becomes very sharp, whereas over the rest of the 

 circuit it is diffuse. And, what is striking, radially outward from that 

 sharp contact is the only place on the sections where the outer growth 

 layer of 1940 has lightwood. The lightwood in itself constitutes a lens. 

 Twelve centimeters outward on the branch, in sections b, the inner 

 growth layers are completely diffuse and the outer growth layer has no 

 lightwood. This situation is duplicated 13.5 cm. farther outward in 

 sections c save for marked fading of the two inner growth layers. The 

 conditions set forth in TTP 24-14 emphasize the hazards of interpre- 

 tations based on one radius (note that a certain radius through 1940 

 on sections a would give one diffuse, two sharp growth layers), based 

 on a single section or for that matter on several sections. 



The sections of TTC 33-20-a give another pertinent example. As is 

 generally true in TTC 33, 1944 has rather weak contacts in compari- 

 son with 1943. The increment for 1944 contains two fairly sharp, 

 complete, entire growth layers plus one fairly sharp, nearly complete, 

 entire growth layer. If we did not know the exact time of cutting 

 (July 31, 1944), if we did not have periodic measurements of tip 

 growth for 1943 and 1944, and if we did not have other specimens 

 also dated exactly for comparative purposes, it vv^ould indeed be dif- 

 ficult to determine which growth layers are annual, if any, and which 

 are intra-annual. 



Variation of marginal contacts in a longitudinal direction is shown 

 in plates 26, 2y, and 28, figure i, where more of the circuit of a growth 

 layer in the 1940 increment becomes diffuse outward on the branch. 



We have found numerous sharp intra-annuals and diffuse annuals. 

 These latter are rare to be sure, we think, but how can one be certain 

 of his interpretations? How is the worker to know whether a bound- 

 ary, sharp around the circuit on several sections, maintains equal 

 sharpness everywhere? If it should be diffuse over a local area, must 

 it be interpreted as intra-annual ? We are confident such a forced in- 

 terpretation no longer is legitimate. Suppose for a moment, however, 

 that a touch of diffuseness did prove intra-annual character in all 

 cases, not just in a majority, then the boundary of every growth layer 

 would have to be examined over its entire area on stem and branch 

 and root. That would be a herculean task calculated to prohibit all 

 interpretative work with growth layers except that of a more re- 

 stricted botanical nature. 



