NO. I GROWTH LAYERS IN TREE BRANCHES — CLOCK ET AL, I27 



within the general growing season, flushes imposed upon the annual 

 cycle. Adding together the fact of postseasonal growth and the addi- 

 tion of an entire growth layer by irrigation of an apple tree in Texas, 

 one becomes firmly convinced that anatomical results are certain to 

 follow if growth factors become sufficiently limiting for a short in- 

 terval during the "growing season." 



Nevertheless, general observations must be substantiated by actual 

 evidence and proof that trees do grow multiple growth layers in a 

 year. Absolute dating is a necessity. For such purpose, these criteria 

 of multiplicity have been employed : natural frost, artificial frost, 

 measured tip growth, certain structural features, and the relation of 

 diameter flushes to tip flushes. 



Examples of dated multiplicity are shown on most of the plates. 

 See especially plates 2, fig. 3 ; 4, fig. i ; 6, fig. i ; 8 ; 10, fig. i ; 14, 

 fig. I ; 15, fig. 2 ; 19, fig. I ; 20; 21 ; 22, fig. 2 ; 23, fig. 2 ; 24, fig. i ; 

 28, fig. 2; 29; 32; 35, fig. I ; 36, fig. I. 



DETAILS OF MULTIPLICITY 



The step immediately subsequent to the recognition of the galaxy 

 of partial growth layers follows logically. It is the great and expected 

 diversity among growth-layer sequences, on different radii of the same 

 cross section, at different levels in the same branch, in different 

 branches of the same tree, in different trees of the same species, 

 among different species, and under different environments. 



Clearly, a knowledge of the exact, and not the assumed, date of 

 each growth layer is of the highest importance to a problem of this 

 kind. Assumptions about one, or more than one, sharply bounded 

 growth layer per year must be eliminated. The idea of a rigidly an- 

 nual unity of growth layers universal in occurrence would be destroyed 

 as such, once multiplicity is established. That done, the fact of unity 

 or multiplicity must be determined for different environments and, 

 where multiple, the incidence or factor of such multiplicity must also 

 be determined. 



How can interpretations based on unity be valid if not unity but 

 multiplicity is present in a region ? There can be no doubt that chron- 

 ologic, climatic, and cycle studies are meaningless until fundamental 

 work on growth layers has been done and the effects of different en- 

 vironments clarified. 



VARIATIONS OF SEQUENCES ALONG DIFFERENT RADII OF A SECTION 



The existence of partial growth layers constitutes a prime reason 

 for the variation of sequences on different radii of the same cross 



