NO. I GROWTH LAYERS IN TREE BRANCHES — CLOCK ET AL. 59 



where on that circuit the lightwood of the growth layer succeeding 

 the partial one makes contact with the densewood of the growth layer 

 preceding it. In a general sense, the term is applicable to the growth 

 layer anywhere over the tree body ; in a restricted sense, it is applied 

 to a cross section but with full knowledge that the characteristics 

 shown on the cross section may, and commonly do, persist in a longi- 

 tudinal direction. 



Simplicity in connection with partial growth layers from a lower 

 forest-border locality is a desideratum not apt to be granted. From an 

 ideal standpoint, a partial growth layer possesses lightwood and dense- 

 wood, both of which fall short of completing the circuit. There are, 

 however, growth layers whose lightwood is partial and whose dense- 

 wood is entire. How should these be classified? Are there partial 

 growth layers which have no lightwood at all ? If so, and if the dense- 

 wood cells of the partial growth layer lie flush against the densewood 

 of the growth layer next interior, then identification would be nearly, 

 or quite, impossible. Could a partial growth layer composed of dense- 

 wood only be located elsewhere, say within the body of a complete, 

 entire growth layer ? The matter depends upon time, rate, and site of 

 cambial activity; upon the interrelationships of one part of the cam- 

 bium to other portions of the cambial sheath ; and upon the rhythm 

 of the growth processes. 



Partial growth layers may be either closed or open. In a closed par- 

 tial, the lightwood covers a shorter arc (or smaller area) than the 

 densewood so that the latter overlaps and extends tangentially (and 

 longitudinally) beyond the lightwood. In an open partial, on the con- 

 trary, the lightwood extends beyond the termination of the dense- 

 wood. 



Types of partial grozvth layers. — Partial growth layers have been 

 subdivided into lenses, half-lenses, temporary lenses, arcs, divided 

 densewood, divided lightwood, interrupted densewood, interrupted 

 lightwood, curtains, and postseasonal growth. 



Lenses are closed partial growth layers ; the lightwood covers less 

 area on the tree body (shorter tangentially on a cross section) than 

 the densewood which always makes contact with the densewood of 

 the growth layer next inside. Stated somewhat differently, a lens con- 

 tains the xylem produced by localized cambial activity. The ideal lens 

 is a patch of xylem of rather complicated geometrical design which 

 has, in general, the complex features of a truncated, partially de- 

 veloped cylinder and cone. In cross section a lens appears concavo- 

 convex, whereas in longitudinal section it is nearly plano-convex. 

 Lenses cover anywhere from zero to lOO percent of the plant body. 



