WOOD. 



[ 821 ] 



WOOD. 



composed of pitted ducts, as is the case also 

 in the common Rose. 



In many of the above trees, the wood 

 acquires a special peculiarity when it attains 

 a certain age; the prosenchymatous cells 

 generally become more solid, year by year, 

 through" the filling-lip of their cavities by 

 the increasing thickness of the secondary 

 deposits on their walls: in the lighter- 

 coloured and softer woods, such as the 

 Lime, there is no distinct line of demarcation 

 between the older and younger part of the 

 trunk the alburnum or sap-wood and the 

 duramen or heart-wood ; but in nanny cases, 

 as in the Ebony (Dwspyros), Lignum vitae 

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

 Oak, &c., the duramen assumes a remark- 

 able solidity and a deeper colour, so that 

 after a certain time the colours of the dura- 

 men 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 o.&Lathrcea, Melampyrum, Cuscuta, &c., 

 there is no layer of spiral vessels corre- 

 sponding with the medullary sheath ; and 

 in the Mistletoe ( 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. 665, 666, 

 page 712). The stem of Myzodendron also 

 exhibits some remarkable anomalies. 



In the Bombacese (Bombax, Carolinea, 

 c.) the mass of structure corresponding 

 to the wood is chiefly composed of mem- 

 branous parenchymatous cells, with scat- 

 tered isolated prosenchymatous cells, and 

 large pitted ducts. The wood ofAvicennia is 

 principally composed of large pitted ducts, 

 with narrow interspaces filled up with small 

 pitted parenchymatous 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 

 have a remarkably "broad spiral-fibrous band 

 (PL 48. fig. 7). The wood of the Casuarinee 

 exhibits a curious structure : it is composed 

 of long prosenchymatous cells, the walls of 

 which, together with those of the numerous 



large ducts, have bordered pits (PI. 48. 

 fig. 2) ; while concentric lines of cellular 

 tissue appear at intervals in the cross sec- 

 tion, consisting of plates of parenchyma 

 extending from one medullary ray to the 

 next, and connecting them. The stems of 

 some of the Menispermaceae have likewise 

 concentric processes of parenchymatous tis- 

 sue. In the WINTEKE^E, a section of the 

 Magnoliaceae, the wood is wholly composed 

 (with the exception of the medullary sheath) 

 of pitted prosenchymatous cells resembling 

 those of Araucaria (PI. 48. fig. 5), without 

 any ducts. 



In certain families of Dicotyledons a 

 remarkable appearance arises from the ar- 

 rangement of the bundles in several circles, 

 almost as in the Monocotyledons ; but this 

 results in a very different kind of structure, 

 on account of the unlimited growth of the 

 cambium in Dicotyledons. Examples of 

 this kind of wood occur in the Chenopo- 

 diaceae, Nyctaginaceae, Piperacese, &c. In 

 Pisonia, which has been supposed to grow 

 I in the same way, the result is a solid mass 

 | of wood, composed of prosenchymatous cells 

 ', and ducts, with isolated perpendicular cords 

 of parenchyma (exactly the reverse of what 

 occnrs in the Monocotyledonous stems). 

 The woods of Phytocrene and Nepenthes 

 may be further cited as offering remarkable 

 peculiarities. 



It would exceed the space which we can 

 allow to this article to enter into a descrip- 

 tion of the anomalous Dicotyledonous stems 

 of the tropical lianes or climbing trees, of 

 the JSignoniacecs, Menispermacece, Malpighi- 

 | acea, &c. ; the irregularities of the wood 

 I of which depend upon deviations from the 

 j normal type arising in the course of the 

 i growth of the stems, which, from the obser- 

 | vations of Treviranus, Criiger, and others, 

 appears to be mostly regular when quite 

 j young. Isolation of one or more fibro-vas- 

 ! cular bundles from the central cylinder of 

 ! wood, producing distinct centres of develop- 

 i ment, is the most common cause of irre^u- 

 larity. 



The wood of Dicotyledons must be exa- 

 mined by transverse sections, and perpendi- 

 cular sections parallel with and at ri^ht 

 angles to the medullary rays. The same 

 applies to the wood of Gymnosperms. The 

 mode of cutting these sections is stated 

 elsewhere. 



Sections of recent woods are best pre- 

 served wet in chloride of calcium. Fossil 

 wood, if silicified, is cut by the lapidary's 



