^ SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I40 



1940 are invisible in their entirety except under high power (pi. 19, 

 fig. I, is suggestive). The increment for 1939 on the cross sections 

 consists at places of one wide and up to two narrow cells radially; 

 elsewhere it consists merely of one row of densewood cells difficult 

 to distinguish from the previous densewood zone except by continuous 

 tracing around the circuit from the place where the lightwood ends. 

 Under low power the increment for 1940 appears to be a long lens, 

 whereas, actually, the lightwood only is lenticular, the densewood 

 being entire. Some radial rows in the portions composed of dense- 

 wood exclusively fail to narrow down, a feature that adds a touch of 

 indefiniteness to the combined bands of densewood. 



In TTP 24-3-a, 1939 is easily visible under low power as a concur- 

 rent lens of two units, whereas under high power it is seen that only 

 lightwood is lenticular and that the densewood is continuous around 

 the entire circuit as a layer two cells thick. No visible boundary 

 separates 1938 from 1939 where their densewoods are in contact. 

 Because of the nature of 1939, the outer margin of 1938 appears to be 

 indefinite except under high power. The outer margin of 1939 like- 

 wise varies from sharp to indefinite, but on the whole it is sharper 

 than that of 1938. The increment for 1939 is, in general, a very thin 

 growth layer; in detail it is, of course, variable in thickness. 



Marked differences in interpretation appear when a growth layer 

 like 1938 above is analyzed under high power. First, reference is 

 made to cambial activity. Low-power analysis would indicate cambial 

 activity completely localized throughout the span of an entire growing 

 season. If the lenticularity is due to a lack of water, as some have 

 supposed, then available soil moisture must indeed have been of 

 nearly negligible amount. Furthermore, such an analysis requires 

 portions of the cambium to forego cell division from the end of one 

 growing season until at least the beginning of the growing season sec- 

 ond removed from the first. There may be a question in some minds 

 that portions of normal cambium can remain in respiratory and met- 

 abolic balance over such long intervals in the presence of photosyn- 

 thesis and without a break in the hydrostatic system. Analysis under 

 high power reveals an increment entire around the circuit and a cam- 

 bium active throughout the same extent. It was active, to be sure, yet 

 so sluggish that it deposited very little lightwood and this was lenticu- 

 lar in distribution. Can the sluggishness be ascribed to any one en- 

 vironmental factor? Other branches of the same tree do not all agree 

 in this lack of high activity. 



The increment for 1939 in TTP 24-3-b (pi. 19, fig. i), 6.5 cm. out- 

 ward from 24-3-a, is somewhat more definite and hence more easily 



