88 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I40 



layer (on one cross section, of course). As indicated, however, the 

 detection of 1940 is not too difficult and, in all probability, it is rep- 

 resented around the entire circuit, being reduced at its minimum to one 

 row of densewood cells. The eye can follow this continuity. Con- 

 stant encounters with growth layers, such as the one for 1940 above, 

 makes one doubt that normal cambium can successfully refrain from 

 cell division throughout an entire growing season. In the other 

 branches of tree TTP 23, the increment for 1940 is highly variable in 

 width, and present either as an entire growth layer or as a lens whose 

 densewood forms part of a band continuous around the circuit. There- 

 fore, in TTP 23-4-a, 1940 is simply reduced to a minimum — reduced 

 to such an extent that its presence could easily pass undetected. The 

 outer portions of the branch (TTP 23-4-b and c) show 1940 much 

 more completely developed, and thus its presence as described in TTP 

 23-4-a is substantiated. The highly variable thickness of the 1940 

 increment, not only among different branches of TTP 23 but also 

 along the extent of a single branch, would make a calculation of 1940 

 volume increment decidedly uncertain and could vitiate or destroy the 

 value of any correlations between rainfall and 1940 tree growth. 

 Even the average thickness on one entire cross section, not to mention 

 a single radius, is apt to give a highly erroneous measure of thickness 

 for the 1940 increment as a sheath over the entire plant body. 



The inner group of the 1933 increment in XSC 1-2-b consists of a 

 thick, entire growth layer, a long exterior lens, and divided dense- 

 wood. Near one cusp of the lens, the densewood of the main growth 

 layer is divided into three narrow bands whose resolution depends 

 upon the use of high power. The outermost band of narrow cells in 

 the multiple divided densewood grades laterally into wider and wider 

 cells which become part of the lightwood at the start of the outer 

 group of growth layers in 1933. Therefore, it is a half-lens which be- 

 gins as divided dehsewood of an inner growth layer and becomes a 

 half -lens in the early part of the next outer growth layer. That the 

 site of a lens is a place where high local stimulation affects the cam- 

 bium is illustrated not only by full compound lenses but also by 

 the less accentuated compounding in which apparently the outer of the 

 lenses is developed merely to the extent of divided densewood of the 

 inner lens, as in the increment for 1934 of XSC i-i-b. 



Amount of magnification is an important factor in the detection of 

 growth layers, in their classification, and in their interpretation as re- 

 gards cambial activity. So-called "low-power" lenses, divided dense- 

 wood, and entire growth layers lacking lightwood over much of the 

 circuit are so closely related they seem to be part of the same feature. 



