STRUCTURE OF THE STEM. 65 
Study the bundles in various portions of the section and notice particu- 
larly whether the relative amount of surface in each covered by ducts and 
by thick-walled wood-cells or sclerenchyma cells is everywhere the same. 
On the whole the structure of monocotyledonous stems is 
much simpler than that of dicotyledonous stems. The bun- 
dles which they contain are somewhat similar to those which 
the exogenous or outside-growing stems of dicotyledons form 
at a very early period of their growth. 
But while in exogens these bundles soon unite into a ring 
of woody tissue, with a cambium layer outside, capable of 
continual growth inward and 
outward, in the endogenous 
or inside-growing stems of 
monocotyledons this is not 
the case. True cambium is 
not formed, but the procam- 
bium which precedes the 
mature bark-cells and wood- 
cells is all transformed into 
cells of bark or of wood, 
which attain their full size 
and are then incapable of 
giving rise to new cells of Susela tg 2 SS 
any kind. Therefore, the FIG. ot) Gkcae bonton of Stem of Indian 
stems of such perennials as ie 
palms remain unchanged in ¢* Hbovasenss boniles #6 nehy me 
diameter year after year. : 
Monocotyledonous stems which do increase in diameter from 
year to year do so by the introduction of new bundles among 
the old ones. This growth by interposition of new bundles 
affords some justification for the name endogenous, often given 
to the monocotyledonous stem. 
88. Distribution of Material in Monocotyledonous Stems. — 
The well-known strength and lightness of the straw of our 
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