NO. I GROWTH LAYERS IN TREE BRANCHES — CLOCK ET AL. 249 



same tendency in the Lubbock area. In sections e, on the long radius, 

 a thick lens of immature cells was being laid down at the time of 

 cutting, November 21, 1940. 



Among the see of YCt 2-5, certain growth layers stand out as more 

 sharply set ofif and as containing a more nearly normal internal se- 

 quence. These correspond to the recorded tip flushes. On the whole, 

 growth flushes and growth fluctuations in the Yuma area appear to be 

 due not so much to temperature as to variations in water supply. 



YCt 3-i-a (193S-1940) 



11 to 13 see + L's + psg — 3 years. 



Unfinished concurrent lenses showed active growth to have been in 

 progress on November 21, 1940, in YCt 3-1 -a. 



YCt 4-1-a {1937-1940) 

 16 see + many L's + psg — 4 years. 



YCt 4-2-a (1937-1940) 



12 see + 5 to 8 L's + Psg — 4 years. 



The actual separation of the growth layers in YCt 4-2 has been 

 given in table 141, page 182. If we summarize the branches of YCt, 

 we find that, on the average, four or more growth layers were formed 

 per year. 



A wholly complete graph of true cambial activity and growth flushes, 

 no matter how transitory the impulse and subdued the effects, or how 

 prolonged the impulse and magnified the effects, would give the only 

 genuine picture of the impact upon the body of the tree not only of 

 the complex of site factors but also of the complex of microsite fac- 

 tors within the larger framework. At present, we can only hope for 

 such a graph. At best, we have recorded such growth layers as see, sL, 

 s arc, s ^L, msce, msL, psce, psL, dee, dL, and the like. Diffuseness 

 in itself varies over a wide range of definiteness and visibility. Hun- 

 dreds of dee's, dL's, and d arcs have passed unmentioned and, no 

 doubt, many more passed unnoticed. Slight evidence of a change in 

 growth activity resides in the faintest of the diffuse growth layers. 

 Interrupted lightwood, interrupted densewood, divided densewood, 

 and divided lightwood also testify to a change of pace in growth 

 activity. Therefore, the growth flushes here recorded and discussed as 

 growth layers include those easily identified and strikingly visible. In 

 contrast to an ultimate graph of growth variations and cambial ac- 

 tivity, the material described herein is, in a manner of speaking, a 

 preliminary portrayal. Nonetheless, it gives a broad hint of the 



