WOODY TISSUE. 45 



cells acquire by incrustation a ligneous consistence and even 

 greater density than wood (39). Nevertheless, the principal 

 and characteristic component of wood in general is thick-walled 

 prosenchyma. So that this takes the name of woody tissue even 

 in the bark, leaves, &c. Fig. 21 represents some pleurenchyma 

 along with the other usual elements of the wood, and shows the 

 manner in which these woody tubes are spliced together, as it 

 were, by their overlapping pointed ends. Their diameter, in this 

 instance, is about the ^(jVo- f an inch. Those of our Linden or 

 Bass-wood (a few of which are shown in Fig. 36, 37) are rather 

 larger, but not more than -j^V^ f an mcn m diameter.* Their 

 size varies in different plants almost as much as ordinary cells 

 do, but they are usually much smaller than parenchyma, espe- 

 cially in herbaceous plants. Perhaps the largest are found in 

 the Pine Family, where they are of a peculiar sort, and are often 

 as much as ^^ or ^^ 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 intermixture of other 

 kinds of tissue, and the thickness of their walls. This is much 

 greater in proportion to their diameter than in ordinary parenchy- 

 ma, and, with their slenderness and their very compact arrange- 

 ment into threads or masses which run lengthwise through the 

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

 terize those parts in which this tissue abounds. A transverse sec- 

 tion under the microscope shows that woody tissue is composed 

 of lengthened cells, that is, of hollow tubes and not of solid fibres 

 (Fig. 20, 36, &c.). But as their walls thicken by the secondary 

 or incrusting deposit to which they are especially liable (39 -41), 

 the calibre diminishes, and in old wood sometimes becomes nearly 

 obliterated. This thickening usually occurs evenly in woody tis- 

 sue ; at least, bands or spiral lines are seldom seen in it ; but 

 small dots or pores, the nature of which has already been explain- 

 ed (44), are not uncommon. They are well shown in the wood 

 of the Plane-tree (Fig. 20-22). Of similar character, only more 

 conspicuously marked, is the 



54. Disc-bearing Woody Tissue (Glandular Woody Tissue of 

 Lindley), which forms the wood in the Pine Family. The nature 



* Lindley states that the woody tubes of the Linden are as much as the 

 Q of an inch in diameter; but I find none of any thing like this size. 



