The Book of Household Management by Mrs. Beeton

system, which consists of another set of small vessels. If, for example, we, early in the spring, cut a branch transversely, we will perceive the sap oozing out from numerous points over the whole of the divided surface, except on that part occupied by the pith and the bark; and if a twig, on which the leaves are already unfolded, be cut from the tree, and placed with its cut end in a watery solution of Brazil-wood, the colouring matter 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.