405. Some parts are built up as a compact structure; in others cells are arranged so as to build up regular air-channels, as in the stems of aquatic and other water-loving plants (Fig. 440), or to leave irregular spaces, as in the lower part of most leaves, where the cells only here and there come into close contact (Fig. 443). [Illustration: Fig. 443. Magnified section through the thickness of a leaf of Florida Star-Anise.] 406. All such soft cellular tissue, like this of leaves, that of pith, and of the green bark, is called PARENCHYMA, while fibrous and woody parts are composed of PROSENCHYMA, that is, of peculiarly transformed 407. =Strengthening Cells.= Common cellular tissue, which makes up the whole structure of all very young plants, and the whole of Mosses and other vegetables of the lowest grade, even when full grown, is too tender or too brittle to give needful strength and toughness for plants which are to rise to any considerable height and support themselves. In these needful strength is imparted, and the conveyance of sap through the plant is facilitated, by the change, as they are formed, of some cells into thicker-walled and tougher tubes, and by the running together of some of these, or the prolongation of others, into hollow fibres or tubes of various size. Two sorts of such transformed cells go together, and essentially form the 408. =Wood.= This is found in all common herbs, as well as in shrubs and trees, but the former have much less of it in proportion to the softer cellular tissue. It is formed very early in the growth of the root, stem, and leaves,--traces of it appearing in large embryos even while yet in the seed. Those cells that lengthen, and at the same time thicken their walls form the proper WOODY FIBRE or WOOD-CELLS; those of larger size and thinner walls, which are thickened only in certain parts so as to have peculiar markings, and which often are seen to be made up of a row of cylindrical cells, with the partitions between absorbed or broken away, are called DUCTS, or sometimes VESSELS. There are all gradations between wood-cells and ducts, and between both these and common cells. But in most plants the three kinds are fairly distinct. [Illustration: Fig. 444. Magnified wood-cells of the bark (bast-cells) of Basswood, one and part of another. 445. Some wood cells from the wood (and below part of a duct); and 446, a detached wood-cell of the same; equally magnified.] [Illustration: Fig. 447. Some wood cells from Buttonwood, Platanus, highly magnified, a whole cell and lower end of another on the left; a cell cut half away lengthwise, and half of another on the right; some pores or pits (_a_) seen on the left; while _b b_ mark sections through these on the cut surface. When living and young the protoplasm extends into these and by minuter perforations connects across them. In age the pits become open passages, facilitating the passage of sap and air.]