6 
remainder, lying immediately below the epidermis, forms a layer of 
bark. Every year a fresh development of tissue takes place between 
the layer of wood and the layer of bark formed the previous year, thus 
adding an additional layer to each of these parts, that to the wood be- 
ing on its outside, whilst that to the bark is on its inside. The old lay- 
ers of bark are necessarily thrust outwards, crack, and decay. Besides 
this arrangement of the woody parts of the stem into concentric zones, 
there are plates of cellular tissue placed vertically, aad extending from 
the centre to the circumference, which in a transverse section appear 
like rays, and form what is termed by artizans the “silver-grain.” The 
stems of Exogene therefore. may be recognized by one or other of the 
following characteristics: 1, A distinct cellular pith, surrounded by 
its vascular medullary sheath; 2, Concentric layers of wood, form- 
ed of both cellular and vascular tissues; 3, Medullary rays of cel- 
lular tissue; 4, A distinct bark of cellular and vascular tissue. 
Although all these characteristics are not necessarily combined in 
every Exogenous stem, they are all wanting in those of the second 
class, or “Endogene.” The latter consist of a homogeneous mass of 
cellular tissue as: Swe Eas the pith ~ an ene — but 
then the vascular 
not collected into a medullary sheath. These bandles of vrédls run nap 
the stem, and then curve towards the outside and enter the leaves. AS 
the stem elongates fresh bundles are formed towards the outside, which 
intersecting the others rise into the leaves of the terminal bud; but the 
stems do not increase much in tniekneess and no cena layers are 
formed. There is no true bark, but th ted by therugged 
bases of the fallen leaves. The i interior continues soft, whilst the exte- 
rior condenses and hardens by the continued addition of new bundles of 
vessels, It was formerly supposed that the new matter always origi’ 
nated towards the nenites and these stems were considered to grow in 
erse of those of the Exogena, from whence 
they obtained the ig of Endogenous (ivdov exvon, inwards, yevvaw 
cENNAO, to beget.) These stems then are recognised by the want of 
the several parts Aaisics in Exoghnoms notions a by the dispersion of 
i] There 
the vascular t 
MSUIC x. 
are no trees of this class in temperate climates, but a good example of 
