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



125 



as environment differs. This ratio, or the incidence of multipHcity per 

 century, can be determined for different regions. 



Multiplicity, as exemplified by partial growth layers, may or may 

 not characterize different parts of the plant body. If partial growth 

 layers should, for instance, characterize branches but avoid the trunk, 

 then we can only conclude that branches give more sensitive records 

 of environmental variations than trunks do. This matter of longi- 

 tudinal continuity has been, and is being, investigated. As far as our 

 work has progressed (chapter VII), the anatomy of growth layers 

 and their multiplicity are very similar in branch and in trunk. 



TYPES OF GROWTH LAYERS INVOLVED 



Multiplicity involves all types of growth layers as well as all types 

 of contacts. The section on classification describes growth layers 



Fig. 36. — XSC 2-2-a. Annual increment for 1936 in symbol and cell structure. 

 Multiplicity by sharp and diffuse gls ; reversed sequence. Sequence shows : Frost 

 effects ; dee with transition outward giving lightest wood in center ; 2 see ; 

 psce; see. 



which range from divided densewood and "stringers," through arcs, 

 half-lenses, and lenses, to entire growth layers, and from incomplete 

 and subnormally developed to complete, normal growth layers. In the 

 matter of contacts the range is equally great, from sharp through 

 definite and indefinite to diffuse (text fig. 36). Each type of contact 

 may apply to a whole growth layer, or all types may exist on the same 

 growth layer. 



