WOOD. 



[ 689 ] 



WOOD. 



(Quercus pedunculata) ; here also the ducts 

 are very large, appearing as open holes to 

 the naked eye in cross sections ; the larger 

 medullary rays are likewise very evident. 

 In the beech (Faaus sylvaticd) there is a 

 small quantity of woody parenchyma, but 

 greatly thickened prosenchyma prevails ; 

 the ducts are rather small, but the broader 

 medullary rays are very evident, appearing 

 as brown streaks to the naked eye in longi- 

 tudinal sections. The Chestnut (Costarica 

 vesca) differs from this chiefly in wanting 

 the broader medullary rays. In the Elm 

 ( Ulmus campestris] the prosenchyma is in- 

 terposed between bands of woody parenchy- 

 ma and wide ducts, which renders the di- 

 stinction of the annual layers obscure. The 

 Walnut tree has no woody parenchyma; 

 the Apple and Pear trees have alternate 

 bands of prosenchyma and woody parenchy- 

 ma; these exist, but are narrower in the 

 Plum and Cherry. In the wood of most of 

 the Leguminosae (Robinia, Vlex, Genista, 

 Gleditschia, &c.) the woody parenchyma 

 appears in bands of considerable size, but 

 the walls of its cells are less thickened 

 than those of the prosenchymatous cells. 

 Woody parenchyma occurs extensively in Ma- 

 hogany and Rose-wood, 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 glutinosd) 

 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 ( Tilia) 

 and the Horse-chestnut (&sculus] have wood 

 of soft texture, the prosenchymatous 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 (Platanus occiden- 

 talis] has strongly marked medullary rays ; 

 the prosenchymatous cells are greatly thick- 

 ened, and mingled with them are very nume- 

 rous ducts, and a small quantity of woody 

 parenchyma. The stem of the Vine ( Vitis 

 mnifera) has likewise long and broad me- 

 dullary rays ; the wood is composed of pros- 

 enchymatous 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 gra- 

 dation of form, from spiral, reticulated, and 

 scalariform to pitted ducts. The various 

 species of Clematis have strongly marked 

 medullary rays, and wood chiefly composed 

 of pitted ducts, as is the case also in the 

 common Rose. 



In many of the above trees the wood ac- 

 quires a special peculiarity when it attains a 

 certain age ; the prosenchymatous cells ge- 

 nerally become more solid, year by year, 

 through the filling-up of their cavities by 

 the increasing thickness of the secondary 

 deposits on their walls: in the lighter-co- 

 loured and softer woods, such as the Lime, 

 there is no distinct line of demarcation be- 

 tween the older and younger part of the 

 trunk, the alburnum or sap-wood and the 

 duramen or heart-wood ; but in many cases, 

 as in the Ebony (Diospyros), Lignum-vitse 

 (Guaiacum), to a less extent in the Elm, 

 Oak, &c., the duramen assumes a remarkable 

 solidity and a deeper colour, so that after a 

 certain time the colours of the duramen and 

 alburnum are very different. This appears 

 to arise from a chemical alteration of the 

 substance of the secondary deposits of the 

 prosenchymatous cells. 



A great degree of regularity and agree- 

 ment of structure exists between the woods 

 of the Dicotyledons above mentioned. It 

 remains to direct attention to various kinds 

 which depart more or less from the type 

 thus selected. 



In the various parasitical Dicotyledons, 

 such as Lathraa, Melampyrum, Cuscuta, 

 &c., there is no layer of spiral vessels corre- 

 sponding with the medullary sheath; and in 

 the Misletoe (Viscum) only annular ducts 

 occur in this situation; the wood in the 

 latter is largely composed of woody paren- 

 chyma, the cells of which are punctated, or 

 possess spiral-fibrous layers (figs. 670, 67 1, 

 page 594). The stem of Myzodendron also 

 exhibits some remarkable anomalies. 



In the Bombaceae (Bombax, Carolinea, 

 &c.) the mass of structure corresponding to 

 the wood is chiefly composed of membranous 

 parenchymatous cells, with scattered isolated 

 prosenchymatous cells, and large pitted 

 ducts. The wood of Avicennia is principally 

 composed of large pitted ducts, with narrow 

 interspaces filled up with small pitted par- 

 enchymatous cells. 



The wood of the Cactaceae, Mammillaria, 

 Melocactus, is composed of dotted ducts, 

 together with a kind of cell, apparently re- 

 ferable to parenchyma, the walls of which 



2 Y 



