analogy which it bears to the veins and arteries in the human system;
but neither it, nor the cellular tissue combined, is all that is
required to perfect the production of a vegetable. There is, besides, a
tracheal system, which is composed of very minute elastic spiral tubes,
designed for the purpose of conveying air both to and from the plant.
There are also fibres, which consist of collections of these cells and
vessels closely united together. These form the root and the stem. If we
attempt to cut them transversely, we meet with difficulty, because we
have to force our way across the tubes, and break them; but if we slit
the wood lengthwise, the vessels are separated without breaking. The
layers of wood, which appear in the stem or branch of a tree cut
transversely, consist of different zones of fibres, each the produce of
one year's growth, and separated by a coat of cellular tissue, without
which they could not be well distinguished. Besides all these, there is
the cuticle, which extends over every part of the plant, and covers the
bark with three distinct coats. The _liber_, or inner bark, is said to
be formed of hollow tubes, which convey the sap downwards to increase
the solid diameter of the tree.