1^ 



SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I4O 



system 4 and 5 that brings up discussion. Previously, the portion 41 

 had been judged to be the half -lens. Now, the separate symbols for 

 4 and 5 suggest a system wherein 4-41-42 is a complete, entire growth 

 layer with an outer margin mostly sharp, partly diffuse, and partly 

 invisible, and wherein 5-51 is a partial growth layer, or lens, additive 

 onto growth layer 4. Growth layer 5, in spite of the lack of dense- 

 wood between 42 and 51, represents an episode of cambial activity 

 which tended to be distinct from the previous one but failed in struc- 

 tural evidence over a portion of the "contact." This adds a new in- 

 terpretation to a half -lens and the cambial activity creating it (see 

 also text figs. 14, 16, 18). How an understanding of cambial activity 

 helps to clarify the classification of growth layers is well shown, 



Fig. 19. — The simple and compound arc. 



therefore, by comparing the figures just discussed with those which 

 follow. 



Temporary lenses by their nature cannot be static forms — they are 

 transitional. Perhaps it is not too much to say that all lenses are 

 temporary for a time. In any event, the term "temporary lens" may 

 well be restricted to a locally stimulated area of the cambium such as 

 out from the base of a twig or branch. All the work being reported 

 upon here emphasizes the fact that cambial division begins locally, 

 thence spreading outward tangentially and longitudinally. The rate, 

 time, and universality of cambial division go far in determining the 

 type of growth layer which results. 



Arcs may be thought of structurally as lenses open at both ends 

 (pis. 3, fig. I ; 16, fig. I ; 33). In reality, the growth layer is recog- 

 nized on a cross section as a band of densewood with both ends 

 "floating" free in lightwood. Text figure 19 shows a simple arc above 

 and a compound arc below. The latter could, of course, be considered 



