CAMBIUM. 2971 
tabular cells, rich in protoplasm, which are in a state of active, 
tangential division. By the activity of this layer vessels are 
separated on the inner side and sieve elements on the outer (Fig. 
138). 
Since this activity is not manifested throughout the entire 
year with the same degree of productiveness, but is more ener- 
getic inthe spring than in autumn, the elements of the spring 
wood are more numerous than those of the autumn wood.’ 
Since in winter the activity of the cambium ceases completely, 
it follows that the large spring cells must directly follow 
upon the narrow autumn cells, There is hereby produced, upon 
> i 
Fie. 138.—Schematic representation of the activity of a cambium-cell (c). 1, before 
the beginning of its activity; 2, the cell has become radially extended, and (8) divided: 
a xylem cell (x’) has been separated. In 4 the cambium-cell is again extended, while the 
xylem cell has already become thickened. 5, the cambium-cell by repeated tangential 
division has this time separated a phloém cell (p’). While the first-formed xylem cell 
(#’) becomes further thickened, the cambium cell is again extended (6), again a tangen- 
tial wall appears within it, and the second xylem cell (a’') and soon afterwards (8) the 
second phloém cell (p’’) is separated. The former become strongly thickened, the 
latter remain thin-walled (Tschirch), 
a transverse section, a visible delineation of rings, for the most part 
concentric, which are designated by the name of annual rings 
roots of dicotyledons, the cambium zone frequently appears as a circular 
line, distinguished by a darker color (Radix Glycyrrhiza, Rad. Calum- 
be, Rhiz. Rhei, Stipites Dulcamare). 
! That the cells become narrower and smaller toward autumn, does 
not proceed (as Sachs, De Vries, and others have stated) from an in- 
creased pressure of the bark in autumn. 
