NO. I GROWTH LAYERS IN TREE BRANCHES — CLOCK ET AL. 37 



site : the cells are narrow radially and possess thick walls which are 

 more or less heavily lignified. This is the ordinary idea of a growth 

 layer. The features enumerated are not always combined, the three of 

 them together. Can one alone establish the identity of either light- 

 wood or densewood? Or are two necessary? Which two? If the cells 

 are thick-walled and heavily lignified but large in radial dimension, is 

 the wood to be called light or dense ? To the unaided eye it would be 

 compression wood. Thick-walled cells, heavily lignified cells, and 

 narrow cells are found isolated, in groups, or in definite patterns any- 

 where throughout a "normal" growth layer ; and the same is true in 

 reverse for the features of lightwood. Since all three do not always 

 occur together, it may be necessary to conclude that the presence of 

 any two of them is sufficient for diagnostic work. A narrow radial 

 dimension, nevertheless, seems to stand out as preeminently character- 

 istic of densewood. 



The ordinary idea of a growth layer also has the lightwood giving 

 way gradually to densewood whose distinctive characteristics become 

 more distinctive outward to the abrupt termination of the growth 

 layer. This is not necessarily universal ; for instance, in a Chisos 

 Mountain juniper (CM J i-i-a), the thickness of the cell walls de- 

 creases outward (see pi. i, fig. 2). Lignification does the same in 

 many instances. In spite of these reversals, the cells gradually become 

 narrower outward and the termination of the growth layer is placed 

 where the narrowest cells lie immediately interior to the very large 

 cells of the next outer growth layer. It seems, therefore, that we have 

 unconsciously but necessarily given more weight to cell size as a diag- 

 nostic feature than to the other two. 



The whole matter goes deeper than merely the physical or visual 

 features of parts of a growth layer. It concerns, of course, the 

 processes of maturation ; more especially, it concerns the vital physi- 

 ological activity of the cambium. Do wall thickening and lignification 

 increase as the speed of cell division in the cambium decreases? Or 

 do the two continue at the same rate that they held during deposition 

 of the lightwood and only appear to increase because the rate of cell 

 division decreases? Are isolated cells or patches of densewood in 

 lightwood, or vice versa, merely "accidents of maturation" and with- 

 out significance in cambial activity? A word must be said about 

 radially narrow cells. Densewood cells gradually become narrower 

 outward until the inner tangential walls of the cells come nearly or 

 quite into contact. Is this decreasing cell width indicative of lessening 

 cambial activity, and does a uniform area of cells with minimum radial 

 dimension mean that the cambium over that area ceased all activity 



