92 GROWTH OF THE BARK, 
spaces. It is this layer which gives to the young bark of trees 
and shrubs their peculiar hues, which are generally brownish 
or some colour approaching to this; or sometimes it possesses 
more vivid tints. In some plants, as in the Cork-oak (fig. 
187, s), this layer becomes excessively developed and forms the 
substance called cork, and hence the name corky layer which 
is commonly applied to it. Large developments of cork also 
occur on some other trees, as various species of Elm. 
On the young bark of most plants may be observed little 
circular or somewhat oval brownish or whitish specks, which have 
been called lenticels (fig. 191, 1,1). They are formed of loosely 
agyregated cork-cells, separated by intercellular spaces, and 
serving, hke stomata, to admit air to the living cortical tissues 
beneath. 
Growth of the Bark.—-The bark, except the middle layer, 
develops in an opposite direction to that of the wood, for while 
the latter increases by additions to its outer surface, the former 
increases by additions to its inner. The bark is therefore 
strictly endogenous in its growth; while the wood is exogenous. 
Each layer of the bark also grows separately ; thus the liber by 
the addition of new matter from the cambium-layer on its in- 
side ; and the phelloderm and corky layer from a special meri- 
stem, which is termed the cork-cambiwm or phellogen. This 
phellogen is placed between the phelloderm and corky layer, so 
that it develops cork-cells on its outside and the cells of the 
phelloderm on its inner surface. The formation of cork-cells, 
however, is not always of the same character, and in some cases 
itis very complex. When the soft tissues of a plant are wounded, 
a callus of cork-cells is also commonly produced, and thus forms 
a protection to the wounded tissues. But when wood is well 
developed, and the plant wounded so deep as the cambium, cork 
is not directly formed, but a callus of parenchymatous tissue is 
produced from all the living cells bordering on the wound. 
The two outer layers which together constitute the paren- 
chymatous or cellular system of the bark generally cease growing 
after a few years, and become dead structures on the surface of the 
tree ; but the inner bark continues to grow throughout the life 
of the individual, by the addition of a new layer annually on 
its inner surface from the cambium. They are commonly so 
thin when separated that they appear like the leaves of a book, 
and hence the supposed origin of the term liber applied to the 
inner bark. The name liber is, however, sometimes considered 
to be derived from the inner bark of trees having been formerly 
used for writing upon. In some trees, as in the Oak, these 
layers may be readily observed up to a certain age; but this dis- 
tinction of the liber into layers is generally soon lost, in conse- 
quence of the pressure to which it is subjected from the growth 
of the wood beneath. 
The outer cellular layers of the site after a certain period in 
