58 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I40 



recognizing that cambial activity has not slowed down perceptibly in 

 the one t3^e and has ceased completely for an interval of time in the 

 other. 



In general practice, the two types, incomplete and complete, are ap- 

 plied specifically to the outermost growth layer of the cross section. 

 Sections cut at different times in the growing season show varying de- 

 grees of completeness in relation to the formation of lightwood and 

 densewood (pis. 4, fig. i; 24; 25; 29; 30, fig. i ; 31 ; 35, fig. i). 

 This is to be expected. Sections cut after the end of the so-called 

 growing season and exhibiting a certain degree of incompleteness on 

 the margin of the outermost growth layer bear directly on the prob- 

 lems of postseasonal growth, of growth layers with indefinite mar- 

 gins, and of growth during what is considered the normal rest period 

 (pis. 10, fig. 2; II, fig. 3; 12, fig. i; 16, fig. 2; 17, fig. i; 19, fig. 

 3). These problems of the outermost growth layer must be 

 recognized in order to understand the reasons for the many types of 

 margins on growth layers when they come to be overlaid by later 

 xylem. Although completeness appears of paramount importance to 

 the most recently formed growth layers, we nevertheless kept accurate 

 record of all growth layers. 



If we turn from the anatomical aspects to the physiological, the sub- 

 ject of completeness becomes highly complex. The cambium neither 

 becomes active as a unit nor rests as a unit, and this holds true no 

 matter what the time of year. Initiation and cessation of activity are 

 not radial exclusively ; they may be tangential, producing lines of de- 

 marcation at an angle between the radial and tangential directions. In 

 this connection, note is made of the densewood of a thick lens. On one 

 radius the cambium may have been at rest, while on another, several 

 radial columns of cells away, it seems to have been dividing actively. 

 A so-called half-lens is the result (pis. 6, fig. 2 ; 22, fig. i ; text 



fig-9)- 



Completeness, in summary, is a radial affair whereas entirety is 

 tangential. The two together aid in giving at least a partial picture of 

 cambial activity in time and in space — in time, at any time during the 

 growing season or during the rest period, and in space, anywhere in 

 the plant body. The two together give us growth layers of great 

 variety, a variety which the present attempt at classification hopes in 

 part to clarify. 



PARTIAL GROWTH LAYERS 



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

 that the growth layer is not continuous around the circuit, that some- 



