will be found to ascend into the leaves and to the top
of the twig. In both these cases, a close examination with a powerful
microscope, will discover the sap perspiring from the divided portion of
the stem, and the colouring matter rising through real tubes to the top
of the twig: these are the sap or conducting vessels of the plant. If,
however, we examine a transverse section of the vine, or of any other
tree, at a later period of the season, we find that the wood is
apparently dry, whilst the bark, particularly that part next the wood,
is swelled with fluid. This is contained in vessels of a different kind
from those in which the sap rises. They are found in the _bark_ only in
trees, and may be called returning vessels, from their carrying the sap
downwards after its preparation in the leaf. It is believed that the
passage of the sap in plants is conducted in a manner precisely similar
to that of the blood in man, from the regular contraction and expansion
of the vessels; but, on account of their extreme minuteness, it is
almost an impossibility to be certain upon this point. Numerous
observations made with the microscope show that their diameter seldom
exceeds a 290th part of a line, or a 3,000th part of an inch.
Leuwenhoeck reckoned 20,000 vessels in a morsel of oak about one
nineteenth of an inch square.

1077. In the vascular system of a plant, we at once see the great
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 wit