THE STEM. 47 
the age of an endogenous stem by the external rings, each of 
which indicates the remains of the leaves developed annually 
at its extremity. This may be seen in the subterraneous 
stem of the Iris pseudacorus or Yellow-water Flag, an indi- 
genous specimen of the endogenous stem. 
147. Stems of this kind are found only in Monocotyledo- 
nous plants ; and the veins in their leaves proceed in parallel 
unbranched lines from the base to the summit, not forming 
a net-work, as in the leaves of the next class. 
Wi. EXOGENOUS STEMS. 
148. The third class of stems consists of those in which 
are observed concentric layers of vascular and woody tissue, 
arranged symmetrically round a central column of cellular 
tissue, enclosed by a hollow cylinder of bark, and covered, in 
annuals, biennials, the annual stems of perennials, and the — 
young stems or shoots of trees, by a cuticle or epidermis. 
There are also rays observed passing from the medulla (the 
central column of eellular tissue) to the circumference, 
termed the Medullary Rays. These parts are well seen on 
_ making a transverse section of a Horse-chestnut, Oak, Elm, 
Plane, Ash tree.—(See Frontispiece, Fig. IL.) 
The different parts to be described in a stem of the Exo- 
genous structure, are 
Tse Ermermtis, 
Tue Hersaceous IntEGuMENT, 
Tue Bark, 
THe Woop, 
' Tue Prrn or Meputia, 
Tae Mepottary Rays. 
Pe in he ee Oe) 
