SECTION 16.] AKATOMICAL STRUCTDRH. 



133 



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 fouud 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 444 445 447 



to be made up of a row of cylindrical cells, 

 with the partitions between absorbed or bro- 

 ken 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. 



409. The proper cellular tissue, or ^«ro«- 

 chyma, is the grouud-vfork of root, stem, and 

 leaves ; this is traversed, chiefly lengthwise, 

 by the strengthening and conducting tissue, 

 wood-cells and duct-ceUs, in the form of 

 bundles or threads, which, in the stems and 

 stalks of herbs are fewer and comparatively 

 scattered, but in shrubs and trees so numer- 

 rous and crowded that in the stems and 

 all permanent parts they make a solid mass 

 of wood. They extend into and ramify in 

 the leaves, spreading out in a horizontal 

 plane, as the framework of ribs and veins, 

 wliich supports the softer cellular portion or 

 parenchyma. 



410. Wood-Cells, or Woody Fibres, 

 consist of tubes, commonly between one and 

 two thousandths, but in Pine-wood sometimes two or three hundredths, 

 of an inch in diameter. Those from the tough bark of the Basswood, 



446 



Fio. 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. 



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 

 i i 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. 



