WOOD. 



Fig. 808. 



[ 820 ] 



WOOD. 



I me c f 



v f c t in. 



Transverse section of Maple-wood three yearo 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 ; 

 I, liber. Magnified 40 diameters. 



of which the former represents sections of a 

 shoot at the begining 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 differ- 

 ence between the structures 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 commences. Here, as is seen, the 

 body of the wood is composed chiefly of 

 prosenchymatous cells (wood-cells or woody 

 fibre,/), with a few pitted ducts (v) near 

 the commencement of each annual layer ; 

 the medullary rays are narrow in this wood. 

 In the Hornbeam (Carpinus Betulus) 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 (Bujcus sempervirens), which is of 

 analogous composition. The Birch (Betula 

 alba) has the same structure. Other com- 

 mon timber-trees exhibit an additional 

 structure in their wood, namely masses 

 woody parenchyma interspersed in various 

 ways among the ordinary prosenchymatous 

 structure of the wood. A very small quan- 

 tity of this occurs in scattered groups in the 

 common Oak (Querctts pedunculata) ; here 

 also the ducts are very large, appearing as 

 open holes to the naked eye in cross sec- 

 tions ; the larger medullary rays are like- 

 wise very evident. In the Beech (Fagu 

 sylvaticd) there is a small quantity of woody 



larenchyma, but greatly thickened prosen- 

 hyma prevails ; the ducts are rather small ; 

 >ut the broader medullary rays are very 

 evident, appearing to the naked eye as 

 }rown streaks in longitudinal sections. The 

 Chestnut (Castanea vesca) differs from this 

 chiefly in wanting the broader medullary 

 ays. In the Elm (Ulmus campestris) the 

 prosenchyma is interposed between bands 

 }f woody parenchyma and wide ducts, 

 which renders the distinction of the annual 

 ayers obscure. The Walnut-tree has no 

 woody parenchyma j the Apple- and Pear- 

 trees have alternate bands of prosenchyma 

 and woody parenchyma; these exist, but 

 are narrower, in the Plum and Cherry. In 

 the wood of most of the Leguminosee 

 (Robinia, Ulex, Genista, Gleditschia, &c.)the 

 woody parenchyma appears in bands of con- 

 siderable size ; but the walls of its cells are 

 less thickened than those of the prosenchy- 

 matous cells. Woody parenchyma occurs 

 extensively in Mahogany and Kose-wood, 4 

 producing a peculiar variation of colour in 

 the wood ; the large holes are the orifices 

 of the very wide ducts. 



The wood of the Poplars (Populus), and 

 Willows (Salix), has the prosenchymatous 

 cells little thickened. The Hazel (Corylus 

 Avellana), and the Alder (Alnus glutinosa), 

 present a peculiarity : the wood appears to 

 the naked eye to have broad medullary rays ; 

 but under the microscope these rays are 

 found to be portions of the wood devoid of 

 ducts, intervening between segments with 

 closely-pitted ducts placed at particular 

 points in the annual rings. The Lime 

 (Tilid) and the Horse-chestnut (^Esculus) 

 have wood of soft texture, the prosenchy- 

 matous cells being only slightly thickened, 

 while the ducts are large and numerous 

 (these exhibit a spiral band, very evident in 

 the Lime). The wood of the Plane (Pla- 

 tanus occidentalis) has strongly marked 

 medullary rays ; the prosenchymatous cells 

 are greatly thickened; and mingled with 

 them are very numerous ducts, and a small 

 quantity of woody parenchyma. The stem 

 of the Vine (Vitis viniferd) has likewise 

 long and broad medullary rays ; the wood 

 is composed of prosenchymatous cells, with 

 a spiral-fibrous deposit on their walls, while 

 the cells of the woody parenchyma are 

 devoid of this ; the ducts are very long, and 

 exhibit every gradation of form, from spiral, 

 reticulated, and scalariform to pitted ducts. 

 The various species of Clematis have strongly 

 marked medullary rays, and wood chiefly 



