NO. I GROWTH LAYERS IN TREE BRANCHES — CLOCK ET AL. 265 



tivity in time and throughout the body of a tree that the resultant 

 xylem representing an entire year's growth covers but a fraction of i 

 percent of the body of a tree (Schulman and Baldwin, 1939). This 

 means, it would appear, that a major portion of the cambium neither 

 died nor set off xylem cells from mid- or late-season of one year until 

 the opening of growth the second year hence ; and in case of a com- 

 pound lens, the third year hence. Several examples could be cited to 

 help explain, at least in part, the problem of a long quiescent cambium, 

 but one will suffice. The increment for 1939 in TTP 24-3-a consists 

 of a series of concurrent lenses when viewed under low power. How- 

 ever, high power shows that the densewood is continuous around the 

 circuit — lightwood is lenticular, densewood entire. The cambium did 

 not fail to divide. At one small locality, the 1940 increment is reduced 

 to two or three rows of densewood cells. Hence, the densewoods of 

 1938, 1939, and 1940 are in contact and would be interpreted under 

 low power as an absence of 1939 and 1940 increments. Adjacent to 

 the locality of merged densewood, the band becomes divided dense- 

 wood as lightwood cells are inserted between the respective dense- 

 woods. 



Nonclimatic causes. — Multiplicity allies itself so naturally with 

 variable factors such as soil moisture, temperature, or defoliation, that 

 it is rather difficult at times to think of genetic causes, the influence of 

 growth hormones, or microhabitat factors. Basically, the influence of 

 heredity and of hormones may be governed by present or past en- 

 vironmental conditions. All are somewhat outside the scope of the 

 present work. 



Any genetic influence which causes multiplicity of growth layers, 

 must, it seems, have arisen at a time when the organism did fit the 

 environment and its growth habits were in some way fixed in the 

 genes. Ultimately, according to modern concepts, terrestrial and cos- 

 mic environment control life and life processes, from the phylum to 

 the species and from the general to the detailed. What we call genetic 

 may, perhaps in some way, be the transmitted control of past environ- 

 mental influences. The situation of the current season is compHcated 

 and the causal factors placed back in time by the formation of a 

 structure in last season's terminal bud which will grow into two tip 

 flushes the following season. Multiplicity of tip growth inherited 

 from terminal buds needs much more study .^° As yet, there is no 



10 According to Hustich (1948, p. 32), Hesselman in 1904 pointed out "that the 

 winter bud consists of the same number of 'short shoots' as that which appears 

 on the 'long shoots' the following summer." 



