NO. I GROWTH LAYERS IN TREE BRANCHES — CLOCK ET AL. 99 



by larger, thinner-walled, green cells. TTC 30-1 combines two types 

 of postseasonal growth. 



In summary, the evidence from a study of postseasonal growth 

 makes it quite clear that a growth layer, even though it be a circle on 

 a cross section, is not necessarily a simple affair ; that tree growth is 

 not restricted to a single pulsation during any one season, or year; 

 and that all the xylem of a growth layer when studied later may not 

 have been formed at the same time as the chief portion of the growth 

 layer. Actual xylem may have been placed at a later date or the 

 growth processess may have gone to completion later than supposed. 

 Computation of the precise amount of xylem formed in the year suc- 

 ceeding the formation of the postseasonal growth would be in error 

 by the amount of that growth, and the thickness of the xylem would 

 yield exaggerated results if compared directly to growth factors op- 

 erating in the year or portion thereof following the formation of the 

 postseasonal growth. 



Some evidence exists to suggest that on occasion the laying down 

 of postseasonal growth is accompanied by terminal bud expansion or 

 even bud elongation. Of the specimens heretofore mentioned, Con T 

 1-18 and those cut off Feburary 21, 1942, bear out the probable corre- 

 lation of the attempted simultaneous growth in diameter and in length. 



SUMMARY OF CLASSIFICATION AND TYPES 

 LONGITUDINAL VARIATIONS 



A classification of growth layers such as here outlined actually rep- 

 resents a synoptic viev/ taken at one locality in a branch at one partic- 

 ular time. Without doubt, growth layers pass from one type to 

 another longitudinally along the branches. A lens may become an en- 

 tire growth layer ; divided densewood may become a lens and then an 

 entire growth layer ; an arc may become a half -lens, then a lens, and 

 finally an entire growth layer ; or vice versa. Lenses may be partially 

 cylindrical "patches," as it were, or an entire growth layer may pro- 

 ject longitudinally as several "fingers," a cross section of which would 

 show the "fingers" as a concurrent lens system. Arcs can be repre- 

 sented by densewood as "floating" in lightwood, or along part of its 

 edge the densewood may join an adjacent band of densewood giving, 

 in cross section, a half-lens. The free edge of a half-lens may drift 

 over to unite completely with adjacent densewood, thus giving a lens 

 in cross section. Text figures 34 and 35 show how the position of the 

 section determines the nature of the growth-layer sequence and how 

 growth layers change from one type to another longitudinally. 



