NO. I GROWTH LAYERS IN TREE BRANCHES — CLOCK ET AL, 57 



annuals actually outnumber the annuals. The increments for 1938, 

 1939, and 1940 on plate 32 contain examples of this type. As was 

 pointed out heretofore, the outer margin of any growth layer can vary 

 around the circuit from sharp through all gradations to highly diffuse. 

 Entire intra-annuals exhibit all these gradations as do true annuals, 

 though the latter do so far less frequently. Workers have applied 

 the terms "false" or "double rings" (Antevs, 1925, pp. 123-124; 

 Douglass, 1928, pp. 31-32; Glock, 1937, p. 10; Hawley, 1941, pp. 

 31-33) to growth layers obviously intra-annual because of their dif- 

 fuse margins. A diffuse "double" is easily detected but this is far 

 from true with sharp doubles. In fact, the whole of our work em- 

 phasizes the impossibility of distinguishing a sharp intra-annual from 

 an equally sharp annual. We can go a step farther. What we have 

 called a thin outer growth layer (pi. 30, fig. 2), terminating the annual 

 increment, commonly possesses densewood so weak in its total de- 

 velopment as to appear indefinite, whereas the inner growth layer — 

 the actual intra-annual— possesses a strongly developed densewood 

 whose outer margin is sharp. The status and significance of this outer 

 thin growth layer will be discussed more at length in the chapter on 

 multiplicity. 



Structurally, an intra-annual consists of an entire circle of dense- 

 wood immersed in the lightwood zone of an annual increment. It may 

 comprise, on the one hand, a densewood zone equal to or more ac- 

 centuated than that of the annual increment itself ; on the other hand, 

 it may be merely a touch of added lignification, slight wall thickening, 

 slight narrowing of the lumens, or a combination of these phenomena. 

 It may be part of an independent, sharply margined growth layer in 

 every sense of the word ; it may be strongly developed on one radius 

 and more or less weakly developed on the opposite ; in either lateral 

 or longitudinal dimension it may run the gamut from strength and 

 sharpness through increasing weakness and dififuseness to total re- 

 placement by the normal lightwood of the annual increment. 



In the study of growth layers a knowledge of the complexities of 

 classification is an absolute necessity, even for a preliminary under- 

 standing of cambial activity. Physiologically, all the types of intra- 

 annual growth layers furnish us a picture of the intricate variations in 

 cambial activity and in the processes of cell maturation. 



Entire growth layers may be separated into two types on their rela- 

 tive completeness. An incomplete growth layer does not have a fin- 

 ished sequence of lightwood and densewood radially. In like manner, 

 then, a complete growth layer does have a finished sequence. These 

 two types may perhaps be translated into physiological functions by 



