WOODY TISSUE. 



43 



as si^ or 2 J^ of an inch in diameter. The density or closeness of 

 grain in wood, however, does not depend so much on the fineness 

 of the wood-cells as ^upon the thickness of their walls. This is 

 much greater in proportion to their diameter than in ordinary 

 parenchyma, and, with their slenderness, and their very compact 

 arrangement into threads or masses which run lengthwise through 

 the stem, conspires to give the toughness and strength wdiich charac- 

 terize those parts in which this tissue abounds. In old wood of the 

 harder kinds, the walls of the cells become so thick as almost to 

 obhterate the calibre (Fig. 198). The thickening is generally uni- 

 form, gi\ang rise to no markings except the pits, or small thin spots, 

 already described (45), which appear like pores. These are of very 

 general occurrence, and are readily seen in the w^ood of the Plane- 

 tree (Fig. 32, a, 46). Markings of this kind are most conspicuous 

 in the Disc-hearing Woody Tissue {Glandular Woody Tissue of 

 Lindley) of the Pine Family, the nature of Avhich has just been 

 explained (46). On account of their markings and their unusual 

 size, and because in the Pine family they make up the wood without 

 any admixture of ducts, these pecu- 

 liar wood-cells have been thought to 

 be rather a form of vascular tissue. 

 But in the Star-Anise much the 

 same kind of marking is found on 

 undoubtedly genuine woody tissue 

 (Fig. 47). In the Yew, on the 

 other hand, where the discs are 

 few, dehcate spiral. markings appear 

 (Fig. 48), showing a transition be- 

 tween the proper woody and the 

 vascular tissues ; as is seen by com- 

 paring the figure with that of a 

 spirally marked duct of Bass-wood, 

 Fig. 50, a. Here the thickening deposit is in two successive and 

 dissimilar layers ; the first, Avith circular vacuities, forming the discs, 

 while the second or innermost bears the spiral markings. 





k 



m 



s 



I 



n 



47 



FIG. 47. Magnified woody tissue of Illicium Floridanum (longitudinal view), marked with 

 large dots, like the discs on the wood-cells of the Pine family. 



FIG. 48. Magnified woody tissue from the American Yew (longitudinal view), some cells 

 showing delicate spiral lines only ; some showing the disc-like markings or dot.s of ordinary 

 Coniferas ; and others with both kinds of markings. Across the base is seen a portion of a 

 medullary ray. 



