220 PHARMACEUTICAL BOTANY 
larger closed collateral bundles in the pith. The pith bundles 
may be more or less uniformly scattered, as in the stems of corn 
and bermuda grass, or arranged in one or more interrupted 
circles, as in Triticum species, etc. The epidermal cells usually 
_ possess cutinized outer walls and frequently all of their walls are 
lignified. ‘The hypodermis contains lignified fibers, and there is 
usually present a sclerenchyma ring directly beneath the endo- 
dermis or the cortex in which bundles may be imbedded or to 
which they may be attached. A sclerenchyma sheath usually 
surrounds each bundle. 
SECONDARY GROWTH IN MonocotyL StTEems.—Secondary 
thickening in stems of monocotyledons is exceedingly rare and 
the exception rather than the rule. It is seen, however, in some 
of the more woody and tree-like genera of the Lily family 
as Aloé, Yucca, Cordyline and Dracaena and in the tuberous Diros- 
-coreacee. In these forms a cambium arises in the inner region of 
the cortex or the pericycle and lays down procambial strands and 
secondary parenchyma on its inner face. As it retreats outward, 
the procambial strands, by repeated division and differentiation, 
develop into secondary collateral (rarely phlocentric) bundles 
which lie imbedded in secondary parenchyma. 
