exceptions, and they are always situated in the interior of cells. Some plants, as many of the cactus tribe, are made up almost entirely of these needle-crystals; in some instances, every cell of the cuticle contains a stellate mass of crystals; in others the whole interior is full of them, rendering the plant so exceedingly brittle that the least touch will occasion a fracture; so much so, that some specimens of Cactus senilis, said to be a thousand years old, which were sent to Kew from South America, were obliged to be packed in cotton, with all the care of the most delicate jewellery, to preserve them during transport.
Besides the cellular tissue, there is a vascular system, which consists of another set of small vessels. If, for example, we, early in the spring, cut a branch transversely, we should 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 ends 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 exuding 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 3000th part of an inch. Leuwenhoeck reckoned 20,000 vessels in a piece of oak 1-19th of an inch in size.
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