THE STEM OF PLANTS. 41 
The dark of trees is essentially composed of fibrous and cellular 
tissue ; but it is easy to understand how varied are the forms, dis- 
position, and structure of these substances, when we consider the 
extraordinary variety in the appearance of the bark of trees, and 
the diversity of their products. To explain everything which 
relates to the structure of the bark, would lead us into details 
which our space does not permit. We must therefore limit our 
remarks, and content ourselves with pointing out the principal 
characteristics of bark, considering generally the trees of our own 
climate. Briefly, the young stem is invariably covered with a 
thin cuticle, the epidermis. As the stem increases, new bundles of 
woody fibres are deposited in regular annular layers one in each 
year, the new layers being deposited outside those already 
formed. The new layers of bark and wood are thus formed almost 
in contact, being juxtaposed. The epidermis covers the bark, as 
it does every other part of the vegetable, but its existence is alto- 
gether ephemeral. It is destroyed at an early stage as much by 
the growth of the vegetable as by the action of external agents. 
It is otherwise with the suber, which forms the next layer, the 
cells of which are of a cubical form, and are closely united to each 
other with thin walls or partitions, without colour at first, but 
afterwards they acquire a brownish colour. 
In many trees the suber is very slightly developed. But this is 
not the case with the Cork-oak (Quercus suber). In this beautiful 
tree, which furnishes man with one of his most useful commercial 
products, the swberous layer acquires an extraordinary thickness ; 
it is, in short, the substance known as cork, in Latin swder, whence 
the specific name of the tree. When about five years old the suber, 
which constitutes the greater part of the bark in the cork-tree, 
begins to make a remarkably quick growth; then all the energy 
of its vegetation seems to concentrate itself on this part of the tree. 
New cells appear on the internal face of the primitive zone, pushing 
the exterior cells which preceded them. Independently of these 
cells, the successive accumulation of which constitute the mass of 
cork, others are formed which are shorter, darker in colour, of a 
flat or plate-like form, which divide the mass of cork into successive 
zones of growth. This mass attains by degrees to a considerable 
thickness. If left to itself, it would crack so deeply as to become 
