THE STEM 51 
closed bundles. The vessels are arranged in a char- 
acteristic manner somewhat after the shape of a V. 
There are usually four. The phloem lies between the 
two largest xylem vessels. Owing to the absence of 
cambium no secondary growth like that occurring in 
dicotyledons is possible. In a few exceptional cases, e.g. 
Yucca, Dracaena, a cambium originates in the pericycle, 
and new bundles are formed, but in a manner quite 
different from that of dicotyledonous stems. Hence the 
thick woody stem characteristic of so many dicotyledons 
is never met with among monocotyledons. 
29. The Stem of the Oak.—The stem of the young 
oak presents a similar appearance to that of the sun- 
flower or broad bean (par. 27) and possesses a large 
central pith, a ring of vascular bundles, and a cortex 
covered by epidermis. But the oak differs from a sun- 
flower in that it is a perennial instead of an annual and 
by subsequent growth the stem assumes a very different 
appearance. The pith may shrink, and instead of 
a ring of scattered bundles we have concentric rings 
of wood. The cortex may disappear altogether, and 
instead of an epidermis we have bark. All these 
changes are brought about by the activity of the cam- 
bium. The first step in this change is the rejuven- 
escence of certain cells between the vascular bundles, 
which cells now form interfascicular cambium. This 
cambium produces new phloem cells on the outside and 
new xylem cells on the inside, so that we get a cylin- 
drical shell of phloem on the outside and a concentric 
cylindrical shell of xylem on the inside (fig. 34). Further, 
one ring of wood is poe each year. Hence each 
