SECTION 16.] 



ANATOMICAL SIKUCTURE. 



133 



tliese, or the prolongation of others, into lioUow fibres or tubes of various size. 

 Two sorts of such transformed culls go together, and essentially form the 



408. Wood. This is found in all common herbs, as well as iu shrubs 

 aud trees, but the former have much less of it iii proportiou to the softer 

 cellular tissue. Ife is formed very early in the grovi'th of the root, stem, 

 aud leaves, — traces of it appearing iu large embryos even while yet in the 

 seed. Those cells that lengtiien, and at the same time thickeu their walls 

 form the proper Woody Fjbue or Wood-cells ; those of larger size and, 

 thinner walls, which are thickened only in certain parts so as to liave 

 peculiar markings, and which often are seen 

 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, ov paren- 

 chymu, is the ground-work of root, stem, aud 

 leaves ; this is traversed, chiefly lengthwise, 

 by the strengthening and conducting tissue, 

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

 bundles or threads, which, in I he stems and 

 stalks of herbs are fewer and comparatively 

 scattered, but in sli>-ubs and trees so uumer- 

 rous and crowded that iu the stems and 

 all permanent parts they make a solid mass 

 of wood. They extend into aud ramify in 

 the leaves, spreading out in a horizon! 

 plane, as the framework of ribs aud veins, 

 which supports the softer cellular portion or 

 parenchyma. 



410. Wood-Cells, or Woody Fibres, 

 consist of tubes, commonly Ijctwecn one and 

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

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



Fio. 444. Mrtgnilioil M-ood-cclls of tlio baric (liast-cells) of Basswood, one and 

 part of anotlier. 44.'). Some wood-cells from tlie wood (and below jiart 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 jiores 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 



