NO. I GROWTH LAYERS IN TREE BRANCHES — CLOCK ET AL, 4I 



FEATURES CAUSING DIFFUSE CONTACTS 



Indefiniteness or diffuseness is caused by two different combinations 

 of features, the one giving irregularity of contact, the other providing 

 a gradual transition between types of wood laid down. Expressed in 

 terms of cambial activity, the first represents variable but complete 

 cessation of that activity among the cambial initials or variable com- 

 pletion of growth among the last xylem cells set off before cessation, 

 whereas the second actually represents a uniform decline of growth 

 activities followed by gradual acceleration with no interval of complete 

 cessation. 



It may be well to describe in some detail the exact anatomical fea- 

 tures, seen on stained sections under a microscope, which detract from 

 the simplicity of growth layers as mirrored so commonly on the 

 natural wood. This will, of course, introduce many complexities not 

 considered ordinarily in work having to do with growth layers, com- 

 plexities that can easily mask the annual identity of a growth layer if 

 sharpness of contact be the only criterion. In practically every in- 

 stance, the use of high power reveals the presence of those features 

 causing diffuseness. Again note should be made that this investigation 

 has to do primarily with branches of trees, most of which grew under 

 extreme lower forest-border conditions. 



Because of the diverse nature of the features causing diffuseness 

 along growth-layer contacts, it is difficult to group them except as ( i ) 

 those along, (2) those interior to, and (3) those exterior to, the con- 

 tact. 



Resin canals in certain instances are disposed along the surface of 

 contact in such fashion as to yield a certain indefiniteness when 

 viewed under low powers. Even moderate power reveals their iden- 

 tity. Parenchyma cells, dark blue or black under staining, occur iso- 

 lated, in groups, or in continuous or discontinuous circles. The outer 

 margin of the inner growth layer of 1939 in TTC 33-13-a (pi. 13) ap- 

 pears indefinite because of scattered parenchyma cells within the 

 densewood and along its outer face. Nevertheless, the contact is sharp 

 entirely around the circuit. The long radius of 1941 in TTC 34-20-a^ 

 bears waves of compression wood some of whose outer borders are 

 far from diffuse. It is a common thing to find such a fairly definite 

 border, whether on compression or on ordinary wood, succeeded im- 

 mediately by a row of dark parenchyma cells. In the case of TTC 



2 Specific examples, rather than general statements, are used to ihustrate the 

 features found along contacts and margins aUhough only a portion of the 

 examples appear in photographs. 



