Il8 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I40 



An interesting case of differential growth exists in TTC 5-4-a. 

 Within the densewood of 1933, a circle of parenchyma cells extends 

 around the circuit at a uniform distance from the inner margin of the 

 growth layer. The circle lies just next to the outer contact of the 

 growth layer on the short radius, whereas on the long radius it lies 12 

 to 15 cells inward from the contact but still within the densewood. 

 Either cambial activity proceeded at a more rapid rate on the long 

 radius after the formation of densewood began or, what may be more 

 likely, activity continued for a greater length of time on the long 

 radius. 



High amplitude waves of cambial activity appear to develop rapidly 

 after the start of the growing season, reach a maximum, and subside 

 either intermittently or continuously. Such activity merely reflects the 

 impact of physiological processes. Examples of time and time rate 

 of activity have been cited heretofore. Growth initiation in the spring 

 in the Lubbock area may occur any time from the first part of March 

 to the first part of April. Within a very short period, growth attains 

 a maximum and then gradually subsides, the whole flush occupying 

 roughly anywhere from about 4 to 12 weeks. Apparently no simple 

 relation exists between elapsed time and number of cells laid down. 

 In one case, several growth layers may be deposited, while in a second, 

 a single growth layer is formed. These variations of time rate apply 

 not only among dift'erent trees but also among different branches of 

 the same tree. 



The following examples may be recalled. The increment for 1941 

 of TTC 34-2-c whose cambium was killed May 18, 1941, contains 2 

 see plus 2 sL plus i inc L. The 1940 increment of TTC 12-io-a, cut 

 April 21, 1940, consists of one to eight cells radially, in one long and 

 one very short lens. In TTC i-io-a (pi. 4, fig. 2), cut April 21, 1940, 

 the same increment consists of 8 to 16 rows of cells around the entire 

 circuit. The same increment in TTC 5-4-a, but cut July 17, 1940, 

 contains 8 to 20 cell rows as an entire growth layer. To say that 

 growth begins on a certain date does not mean that the entire cam- 

 bium becomes active at the same time. The position of natural frost 

 injury, or a circle of parenchyma cells, is an excellent example (pis. 8 ; 

 14, fig. i). The injury may be out in the xylem 7 to 10 or more cells 

 from the start of the growth layer ; it may be out a large number of 

 cells on the long radius but lie flush against the densewood of the pre- 

 vious growth layer on the short radius ; or it may be out in the xylem 

 on the long radius, pull inward on both sides of that radius, meet the 

 densewood of the previous growth layer, and cease (pi. 22, fig. i). 

 Thus in the last case a decided lens had been formed before the cam- 



