WOOD. 



[ 688 ] 



WOOD. 



down in a kind of false cambium layer be- 

 neath the rind. 



Interesting objects illustrating the above 

 structures are furnished by longitudinal and 

 transverse sections of the trunks of large 

 Palms and of the large woody leaf-stalks of 

 these, of canes of different kind, of Bamboo- 

 canes, the rhizome of Sarsaparilla-plants 

 (Smilax), Ruscus, the harder parts of the 

 stem often found attached to imported Pine- 

 apples, &c. Sections of silicified fossil 

 Palm-stems, prepared by the lapidary, can 

 also be obtained from the dealers in objects. 



Dicotyledons. In this class we meet with 

 a remarkable diversity in the character of 

 the wood, which moreover here exhibits, 

 from the indefinite power of growth of the 

 FIBRO-VASCULAR BUNDLES, a much more 

 extensive and perfect development than in 

 the Monocotyledons. In the articles ME- 

 DULLA (fig. 459, p. 419), MEDULLARY 

 RAYS (fig. 461, p. 419), and VASCULAR 

 BUNDLES (fig. 797, p. 669) are described the 

 conditions of ordinary Dicotyledonous stems 

 in the first year of their growth ; it is stated 

 in the account of the vascular bundles, that 



Fig. 812. 



Transverse and vertical section of a segment of a shoot 

 of the Maple in the early part of the second year of its 

 age. T, spiral vessels ; V P, pitted ducts ; F, woody 

 fibre ; C, cambium ; PC, cortical parenchyma ; F, liber 

 fibres ; PC, cellular envelope of the bark ; S, corky layer 

 of ditto. Magnified 60 diameters. 



a new layer of wood is developed in the 

 cambium layer in each succeeding season 

 (fig. 461, p. 419). The nature of the ele- 

 mentary structures in such cases is illustrated 

 by the accompanying figures from the Maple 

 (Acer campestns) (812 & 813), of which 



Fig. 813. 



Transverse section of a Maple-wood three years old. 

 The figures 1, 2, 3, indicate the annual rings of wood, the 

 rest is bark, m, medulla ; t, spiral vessels ; v, ducts, /, 

 woody fibre ; c, cambium ; me, medullary parenchyma ; 

 /, liber. Magnified 40 diameters. 



the former represents sections of a shoot at 

 the beginning of its second year, when the 

 cambium layer (c) is swelling ; the latter a 

 shoot of three years' growth, the portions 

 belonging to each year being indicated by 

 the figures. The only difference between 

 the structure developed in each succeeding 

 season is the absence of a layer of spiral 

 vessels (medullary sheath, in the first year) 

 at the point where each year's growth com- 

 mences. Here, as is seen, the body of the 

 wood is composed chiefly of prosenchyma- 

 tous cells (wood-cells or woody fibre) (/), 

 with a few pitted ducts (v} near the com- 

 mencement of each annual layer ; the me- 

 dullary rays are narrow in this wood. In 

 the Hornbeam (Carpinus Eetulus] the wood 

 is of very similar composition; the wood- 

 cells, however, are more thickened, and the 

 ducts exhibit a spiral marking ; the annual 

 layers are not very clearly defined in sections 

 under the microscope. This is the case, 

 again, with the excessively hard wood of the 

 box (Buxus sempervirens)., which is of analo- 

 gous composition. The Birch (Betula alba] has 

 the same structure. Other common timber 

 trees exhibit an additional structure in their 

 wood, namely masses of woody parenchyma 

 interspersed in various ways among the or- 

 dinary prosenchymatous structure of the 

 wood. A very small quantity of this occurs 

 in scattered groups in the common oak 



