WOOD. 



[ 821 ] 



WOOD. 



composed of pitted ducts, as is the case also 

 ill the common Rose. 



Ill many of the above trees, the wood 

 acquires a special peculiarity when it attains 

 a certain age ; the proseiichymatous cells 

 generally 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- 

 coloured and .-softer woods, such as the 

 Lime, there is no distinct line of demarcation 

 between the older and younger part of the 

 ti'unk — the alburmun or sap-irood and the 

 duramen or Jieart-n-ood ; but in mnny cases, 

 as in the Ebony (Dwspjjros) , Lignum vitfe 

 (Guai'acum), to a less extent in the Elm, 

 Oak, Szc, 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 prosenchymatons cells. 



A great degree of regularity and agree- 

 ment of structm-e 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 &.S, Lathrcea , Melampyrum, Cuscuta, &c., 

 there is no layer of .spiral vessels con-e- 

 sponding vdth the medullary sheath ; and 

 in the Mistletoe ( Viseuni) 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 J/yror?? »f/ro« also 

 exhibits some remarkable anomalies. 



In the Bombacese (Bombax, Carolinea, 

 kc.) the mass of structure corresponding , 

 to the wood is chiefly composed of mem- j 

 branous parenchymatous cells, with scat- 

 tered isolated prosenchymatons cells, and '• 

 large pitted ducts. The wood of Avicentiia is i 

 principally composed of large pitted ducts, 

 with naiTow mterspaces filled up with small 

 pitted parenchymatous cells. 



The wood of theCactacefe {MammiUaria, 

 Melocadus) 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 Casiiarincs 

 exhibits a cm-ious structure : it is composed 

 of long prosenchymatons 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 WiNTKEE^, a section of the 

 Magnohaceae, the wood is wholly composed 

 (with the exception of the medullary sheath) 

 of pitted prosenchymatons 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 veiy 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, Nyctag-inaceae, Piperacefe, &c. In 

 Pisonia, which has been supposed to grow 

 in the same way, the result is a solid mass 

 of wood, composed of prosenchymatons cells 

 and ducts, with isolated perpendicular cords 

 of parenchyma (exactly the reverse of what 

 occurs 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 Bignoniace(S, 3Ienispermacea;, Malpighi- 

 ace(^, Sec. ; the irregularities of the wood 

 of which depend upon deviations from the 

 normal type arising in the course of the 

 growth of the stems, which, from the obser- 

 vations of Treviranus, Crtiger, and others, 

 appears to be mostly regular when quite 

 young. Isolation of one or more fibro-vas- 

 cular bundles from the central cylinder of 

 wood, producing distinct centi-es of develop- 

 ment, is the most common cause of irregu- 

 larity. 



The wood of Dicotyledons must be exa- 

 mined by transverse sections, and pei-pendi- 

 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 



