1921] CHAM BERLAIN—GROWTH RINGS 299 
Another peculiar feature of the bundle with disorganizing 
_ phloem is the appearance of vigorous meristematic activity in 
the cells surrounding the xylem. These cells behave like a cam- 
bium, so that rows consisting of as many as eight cells may be 
formed (figs. 4, 5). If differentiation should take place, we should 
expect to find a xylem zone and, perhaps, phloem surrounding 
the primary bundle; but 
development stops soon after 
the stage shown in fig. 5, be- 
fore any lignification can be 
detected. The distribution 
and general appearance of the 
primary bundles are shown in 
fig. 6. 
That the secondary growth 
in some monocotyls, like 
Yucca, Dracaena, and Aloe, 
results from meristematic 
activity is well known. The 
piece of a transverse section 
of Aloe ferox, natural size 
(fig. 2a), already referred to, 
and a somewhat magnified 
view of the origin of second- 
ary structures (fig. 2), show 
the position of the structures 9 10 
to be described. The phel- Fics. 7-10.—Alve pleuridens: four early 
logen is evid ently hyp odermal 528° in development of secondary bundle; 
in origin, and it builds upa 
limited amount of secondary cortex, with rectangular cells in 
regular rows abutting upon the smaller spherical cells of the 
primary cortex. The cambium which gives rise to the vascular 
structures is pericyclic in origin, and, as seen in transverse section, 
gives rise to long rows of cells. The cells on the outer side of the 
cambium undergo comparatively little differentiation; they enlarge 
to about twice the size of the cambium cells, and many of them 
become almost entirely filled with needle-shaped crystals of calcium 
