THE STEM. 37 
_ two being separated by what may be called fascéeular cambium, 
since it occurs within the bundle. In a rather older stem some 
of the cells making up the medullary rays have begun to divide 
actively, giving rise to cambium between the primary bundles 
or znterfascicular cambium. This, together with the fascicular 
cambium, forms a 
cambium ring, ex- 
tending right round 
the stem. This and 
even a later stage is 
reached in the stouter 
stems of sunflower. 
Still older examples 
present a compact 
mass of vascular tis- 
sue, most of which 
has been formed by 
the active division of 
the cambium, produc- 
ing secondary bast on 
the outside and secon- 
dary wood on the in- 
side, not only in the 
region of the primary 
bundles, but also be- 
tween them. ‘The 
elements making up 
the new bast and 
wood are similar to 
those already de- 
scribed, but spiral 
and annular air tubes 
are limited to the 
protoxylem. Secon- 
dary increase is far 
greater in the case rele 
she Fic. 8.—Secondary Thickening of Stem. [After Sachs.] A, 
of the xylem, giving B,C. cross sections of same stem at different ages. A. 
rise to the “ wood ”’ young bundles quite separate from one another. B. a 
cambium ring present. C. a cylinder of secondary xylem 
of shrubs and trees, and phloém present, formed by activity of cambium ring; 
= ‘cc a2 M. pith; R. cortex ; medullary rays in A and B are the 
while the bark” is broad spaces between bundles, in C are represented by 
largely composed of dark lines; p, ‘fp, phloém; a, fh, ifh, xylem. 
secondary bast. The primary medullary rays are reduced 
to exceedingly narrow strips, running through from pith to 
cortex. Secondary or short medullary rays are also formed, 
taking origin in the cambium, and only extending part of 
