The Book of Household Management by Mrs. Beeton

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.