30 ORGANOGRAPHY. 



leave no interspaces {fig. 55). The woody portions of all plants 

 consist in part of this form of tissue. It is also found in the 

 liber or inner bark mixed with parencliyma, and in the veins 

 of leaves and those of other membranous expansions of the 

 stem or its divisions. 



We distinguish three kinds of cells Avhich enter into the 

 composition of Woody Tissue; namely, the ordinary Wood- 

 cells, the Disc-bearing or Punctated Wood-cells, and Liber-cells ; 

 these form respectively, by their combination, the ordinary 

 Woody Tissue, the Disc-beaiing ox Punctated, andWoody Tissue 

 of the Liber. 



a. Woody Tissue. — This, the ordinary kind of woody tissue, 

 is composed of cells the sides of which, although thickened 

 by secondary layei's, either present a homogeneous appearance, 

 as is more commonly the case (fig. 55), or are marked with 

 little dots or pits, as in pitted cells. The occurrence of spiral 

 fibres, or rings, or reticulations, is exceedingly rare in wood- 

 cells. The secondary deposits are arranged in concentric 

 layers, which increase in number as the cells progress in 

 age, so that in old wood their cavities are often nearly oblite- 

 rated (fig. 54). This kind of tissue occurs in the wood of most 

 trees, except that of the Coniferai and allied orders ; and in the 

 veins of leaves, &c. The peculiar manner in which these wood- 

 cells are arranged with respect to each other, overlapping at 

 their pointed extremities, and thus becoming tirmly cemented, 

 as it were, together, combined Avith the thickness of their walls, 

 renders this tissue very strong and tough, and thus admirably 

 adapted for those parts of plants in which it is found, and 

 where such qualities are especially required. 



b. Disc-bearing Woody Tissue. (Glandular of Lindley^ 

 Punctated of some authors.) — This tissue is composed of those 

 wood-cells called Disc-bearing Wood-cells, which liave been 

 already described. (See page 13.) This tissue constitutes gene- 

 rally nearly the whole of the wood of the Coniferaj and allied 

 order Cycadacea, as well as a portion of the wood of some other 

 plants, as Wintera aromatica, Illiciumfloridanunu &c. The disc- 

 bearing wood-cells, hoAvever, in the latter cases, are somewhat 

 modified, the discs being here found witliout the central ])it or 

 dot (fig. 24). These disc-bearing wood-cells are much larger 

 than the otlicr kinds, being often as much as ■— or — of 

 an inch in diameter, while the latter are frequently nof more 

 than v~^, or on an average about ■— j^ of an inch in dia- 

 meter. 



c. Woody Tissueof the Liber. (Bast Tissue of some authors.') — 

 This consists of cells much longer than ordinary wood-cells 

 (fig. 57), with very thick walls (fig. 58), and tougher, but at 

 the same time softer and more ficxihle. Hence these are a 

 peculiar kind of cells, and have received the distinctive name 



