52 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I40 



of growth-layer contacts. The sections for TTC 35-7 were cut July 

 31, 1944. Sections a were taken 23.4 cm. inward from tip, sections b 

 17.2 cm., and sections c 12.2 cm. In sections a, the increment for 1944 

 contained three sharp, complete, entire growth layers. In sections b, 

 the contacts of the three growth layers had deteriorated considerably 

 — they classified as indefinite. Out in sections c the three growth 

 layers could be identified chiefly because they had been previously 

 identified in slide a. No actual densewood cells were present ; larger, 

 thinner-walled cells simply follow radially on thicker-walled cells with 

 little change in cell size. Sharply defined growth layers disappear out- 

 ward on the branch. 



The amount of magnification determines to such a decided extent 

 the interpretation placed upon the results of cambial activity that one 

 wonders whether he dare trust either the unaided eye or very low 

 powers. Perhaps the eye or very low power reveals only the grosser 

 steps in the life record of a tree and misses those more intimate de- 

 tails which actually constitute the life history of a tree or community 

 in success or failure and which we so greatly desire to know. In any 

 event, it happens that the lower the power used, the greater the num- 

 ber of contacts interpreted as sharp. Features of the densewood 

 which detract from sharpness fade under lower powers. The lower 

 the power, the more the eye depends upon contrasts of color and con- 

 trasts of light reflection. If, on the contrary, normal densewood is 

 followed by atypical lightwood and the contact appears diffuse to low 

 power, high magnification alone reveals the actual sharpness of con- 

 tact, especially so on stained sections. In the case of Con T i-io-a, the 

 wide-field binocular shows three normal growth layers whereas the 

 microscope reveals that one of the growth layers is diffuse over most 

 of its extent. 



VARIABLE CONTACTS 



Contacts have been described as either sharp, definite, indefinite, or 

 diffuse, which for the sake of simplicity could be grouped as sharp or 

 diffuse, and attempts have been made at definitions. In descriptions 

 of actual specimens, however, it becomes apparent that a single con- 

 tact as seen on a cross section can and does vary from sharp to diffuse. 

 Variation also occurs in the longitudinal direction. In other words, 

 types of contacts can occur singly or in combination. The general 

 problems arising from variations and combinations are probably best 

 illustrated by specific examples. 



The increment for 1940 in TTP 24-14-a (pi. 19, fig. 2) contains 

 three growth layers, the inner one diffuse, complete, and entire, the 



