56 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I40 



not too plentiful, where high power alone reveals the true sharpness of 

 a contact. 



Our work amply shows that weakness of contact does not avoid an- 

 nual boundaries. Assuredly it would lend simplicity to the matter if 

 sharpness were restricted to the outer borders of annual increments 

 and diffuseness to intra-annuals. Extensive work may ultimately 

 prove that an intra-annual will break down from rigid sharpness 

 somewhere in its total area. One could perhaps be excused for the 

 hope that this will be proved true. Nevertheless, any such startling 

 simplicity seems at present to be ruled out — a percentage of annual 

 contacts shows weaknesses, and a percentage of intra-annuals shows 

 strength. The conclusion is verified by absolute dating. 



CLASSES AND TYPES OF GROWTH LAYERS 



Classification is here considered as a display of the various ana- 

 tomical forms presented to us by growth layers in the xylem. Such a 

 more or less orderly arrangement is considered merely a first step, an 

 adjunct, in the complex problem of cambial activity. Only when we 

 learn something of the intricate nature of cambial activity, its time of 

 occurrence, rate and place of cell division, and interrelationships, can 

 we hope to inquire into the environmental factors which directly and 

 indirectly, separately and combined, influence that activity. 



Growth layers are classified as either entire or partial. 



ENTIRE GROWTH LAYERS 



Definition. — The term entire as applied to a growth layer signifies 

 that the growth layer is continuous around the circuit of the tree sec- 

 tions under study, that nowhere on the circuit does the lightwood of 

 the growth layer succeeding it make contact with the densewood of 

 the growth layer preceding it. The term is, of course, equally appli- 

 cable to the presence of the growth layer over the whole body of a 

 tree. However, in this report it is used in connection with the branch 

 or portion thereof under study at the time. Examples are shown in 

 plates I, figure i ; 8; lo, figure i ; and 14, figure 2. 



Types of entire growth layers. — The annual, in which one entire 

 growth layer constitutes the annual increment, is probably the most 

 common type of growth layer in certain geographic regions. In plate 

 I, figure I, the growth layer for 1938 is very close to being a single, 

 entire, annual growth layer (see also pi. 2, fig. 3). 



The intra-annual type is less common than the annual except in 

 some trees of certain regions and in some years, where the intra- 



