192 STEMS 
Closed vascular bundles and their scattered arrangement are 
the chief distinguishing features of the anatomy of monocotyle- 
donous stems. 
Structure of Herbaceous Dicotyledonous Stems 
Herbaceous Dicotyledons constitute an important group, for 
they include many forage plants, notably the Clovers and Alfalfa, 
some important fiber plants as Flax and Hemp, most vegetables, 
and many greenhouse plants. In the tropical countries there 
are a few Gymnosperms that are herbaceous, but in general 
features their anatomy is 
quite similar to that of 
herbaceous Dicotyledons. 
All stems of the herba- 
ceous dicotyledonous type, 
whether they are stems 
strictly herbaceous through- 
out or only the young 
branches of woody stems, 
have pith, vascular cylinder, 
and cortex which occupy well 
separated regions when well 
Fic. 171. — Diagram of a cross section developed. Cross sections 
of a well developed herbaceous stem, show- appear to the naked eye 
ing the epidermis (a); band of tissue (}) about as shown in Figure 
composed of cortex and phloem; xylem 777. The epidermis, cortex, 
cylinder to); Bnd pith (2), and phloem form the soft 
outer zone, while the xylem forms the woody cylinder just within 
the soft zone, and encloses the pith, which occupies the center of 
the stem. In order to trace the development and study the 
anatomy of the different tissues, we must turn to highly mag- 
nified sections as shown in Figure 172. 
The Cortez, which is the larger part of the outer zone of tissues, 
is covered by the epidermis, and includes the starch sheath as its 
innermost layer. Just under the epidermis some of the cells of 
the cortex are transformed into collenchyma cells, which are par- 
ticularly abundant in the angles of the stem shown in the Figure 
but more generally distributed around the stem in many other 
plants. The collenchyma cells, often noticeable in sections on 
account of their whitish glistening appearance, have much thick- 
