THE STEM 215 
Hisrotocy or TypicaL Hrersactous Monocoryt STEMs 
(ENDoGENous).—Passing from exterior toward the center the 
following structures are seen: 
1. Epidermis whose cells are cutinized in their outer walls. 
2. Hypodermis, generally col- a 
lenchymatic in young, terminal 
portions, becoming later scler- 
enchymatous. 
Fic. 144. Fic. 145. 
Fic. 144.—Photomicrograph of cross-section of very young cornstalk (a herbace- 
‘ous monocotyl stem), where certain plerome strands have just gone over into 
vascular bundles. For comparison with Fig. 145. (Stevens.) 
Fic. 145.—Photomicrograph of cross-section of older cornstalk stem; a, epi- 
dermis; 6, cortex and c, ground tissue of pith. (After Stevens.) 
3. Cortex, of cortical parenchyma cells. - Sometimes vascular 
bundles of the closed collateral or the concentric type occur in 
this region. 
4. Endodermis or innermost layer of cortex and Pericycle 
(frequently appearing like and often indistinguishable from corti- 
cal parenchyma cells). 
5. Stele or Pith, a large central zone of parenchyma matrix in 
which are found scattered fibro-vascular bundles of the closed 
collateral or more rarely of the concentric monocotyl (phlocentric) 
type. In this latter type, which is typical of old monocotyl stems 
and also seen in Calamus, Convallaria, etc., the xylem grows 
completely around the phloem so that phloem is found in the 
center and xylem without and surrounding it. 
HisroLocy oF A TypicaL MonocotyL Ruizome.—The 
rhizome of Convallaria majalis (Lily-of-the-Valley) shows typical 
herbaceous monocotyl rhizome structure. A transverse section 
