80 EXOGENOUS STEM.—PITH. 
of trees and shrubs, at first resemble those which are herbaceous 
or die yearly, except that the wood in such plants is generally 
firmer and in larger proportion. As growth proceeds in the 
second year, a new ring of wood is formed on the outside of 
the one of the previous year (fig. 183, 2), while at the same 
time a new fibrous layer is added to the inside of the bark, /. 
These layers are developed out of the cells of the cambium 
layer, already alluded to as being situated between the xylem. 
and the phloém of the indefinite fibro-vascular bundles which 
form the stems of Exogenous plants (figs. 179, c, and 180, c). 
The medullary rays (fig. 185, A, 7, 1), at the same time increase 
by addition to their outside, and thus continue to keep up the 
connexion between the pith and the bark. In_ succeeding 
years we have in hike manner new layers of wood and liber, 
one of each, as a rule, for every year’s growth (fig. 183, 3), while 
the medullary rays also continue to grow 
Fia. 184. from within outwards. Hach succeeding 
year’s growth is therefore essentially a re- 
petition of that of the first year, except as 
regards the pith and spiral vessels ; the former 
of which does not increase in size after the 
first year, and the latter are never repeated, 
so that in old stems we have no more dis- 
tinct regions than in those of the first year. 
We have consequently in all Exogenous 
stems the following parts, namely, pith, 
medullary sheath, wood, medullary rays, cam- 
bium layer, and bark—which we shall now 
describe in the order in which they are placed. 
1. Pith or Medulla (figs. 182, m, and 185, 
B, a, a).—This consists essentially of paren- 
chyma, and it forms a more or less cylin- 
drical or angular column which is situated 
commonly at, or towards, the centre of the 
Fig. 184. Young stem. As a general rule the pith is not 
branch of Walnut continued into the root, but it is always in 
Cae connexion directly with the terminal bud 
the discoid pith. of the stem, and also at first indirectly by 
the medullary rays with all the lateral leaf- 
buds ; as the latter, however, continue to develop, their con- 
nexion with the central pith is cut off, as will be explained 
hereafter in speaking of their structure and origin. The paren- 
chyma of which the pith is composed is generally that kind 
which is known as regular (fig. 65), so that when a section is 
made of it, and examined microscopically, it presents an hex- 
agonal (figs. 66 and 185, B, a, a) or polyhedral appearance. 
In the earliest stages of the plant’s existence the whole of it 
consists of parenchyma ; and out of this tissue, by the differen- 
tiation of special cells, the more elaborate spiral and other 
