NO. I GROWTH LAYERS IN TREE BRANCHES — CLOCK ET AL. 267 



Some of the facets on the subject of muItipHcity as related to chro- 

 nology are here outlined. 



Anatomical problems. — In the final analysis, certain intra-annuals 

 cannot be distinguished from annuals. Many of the intra-annuals, 

 within the limits of visible area, possess a border which is diffuse any- 

 where from a small arc to an entire circuit. Such localization of dif- 

 f useness exists also in the longitudinal direction. Other intra-annuals, 

 within the limits of visible area, possess a border which is entirely 

 sharp and hence exactly resembles that of an annual growth layer. It 

 may be that somewhere within the body of a tree, no matter how small 

 the area may be, the margin of every intra-annual breaks down into 

 diffuseness. If this should turn out to be true, many cases would be 

 beyond our reach because of the impossibility of dissecting completely 

 the entire body of each tree under study. 



Annual contacts, furthermore, are not always sharp. In any one out 

 of the following ii ways, the dense wood of the annual may have sub- 

 sided from a definition of utmost sharpness. 



(i) Localized areas of cells on the outermost part of a growth 

 layer, or more or less alternate columns or groups of columns in the 

 densewood region, resemble lightwood cells. 



(2) The outer margin of the growth layer has been made irregular 

 because alternate columns of tracheids protrude into the zone of the 

 next outer growth layer. 



(3) Scattered parenchyma cells may not only make a margin ir- 

 regular but also impair definition. 



(4) Postseasonal growth commonly causes an irregular, indefinite 

 margin. 



(5) Immaturity of outer cells of densewood produces a diff useness 

 which is striking under low magnification and difficult or impossible 

 to resolve on unstained sections. 



(6) Typical densewood fails to form. In TTC 35-7, for instance, 

 sections a taken 23.4 cm. from tip of branch, show 3 see for 1944; 

 sections b, at 17.2 cm. from tip, show 3 rather indefinite ce growth 

 layers; and sections c, at 12.2 cm. from tip, show 3 diffuse ce growth 

 layers. In sections c, no normal densewood cells were present — 

 larger, thinner-walled cells simply follow thicker-walled cells radially 

 with little change in cell size. TTE i-io-a, taken 60 cm. from the tip, 

 gave an interesting situation in that the 1940 increment is dominated 

 by vessels and has no definite densewood of small polygonal tracheids, 

 whereas the 194 1 increment is dominated by tracheids and does have 

 a densewood of tiny irregular tracheids. If the growth layers had not 

 been dated, the 1940 and 1941 increments together would no doubt 



