AUU 



WOOD. 



[ 690 ] 



WOOD. 



have a remarkably broad spiral-fibrous band 

 (PI. 39. fig. 7)- The wood of the Casuarince 

 exhibits a curious structure ; it is composed 

 of long prosenchymatous cells, the walls of 

 whioJi, together with those of the numerous 

 large ducts, have bordered pits (PL 39. fig. 2), 

 while concentric lines of cellular tissue ap- 

 pear at intervals in the cross section, con- 

 sisting of plates of parenchyma extending 

 from one medullary ray to the next, and 

 connecting them. The stems of some of the 

 Menispermacese have likewise concentric 

 processes of parenchymatous tissue. In the 

 Wintered, a section of the Magnoliacese, 

 the wood is wholly composed (with the ex- 

 ception of the medullary sheath) of pitted 

 prosenchymatous cells resembling those of 

 Araucaria (PI. 39. fig. 5), without any 

 ducts. 



In certain families of Dicotyledons a re- 

 markable 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 Chenopodia- 

 cese, Nyctaginacese, Piperacese, &c. In Pi- 

 sonia, which has been supposed to grow 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 

 occurs in the Monocotyledonous stems). 

 The woods of Phytocrene and Nepenthes may 

 be further cited as offering remarkable pe- 

 culiarities. 



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 families Bignoniacece, MenisjjermacecB, 

 MalpighiacecB, &c., 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, Criiger, and others, 

 appear to be mostly regular when quite 

 young. Isolation of one or more fibro-vas- 

 cular bundles from the central cylinder of 

 wood, producing distinct centres of develop- 

 ment, is the most common cause of irregu- 

 larity. 



The wood of Dicotyledons must be exa- 

 mined by transverse sections and perpendi- 

 cular sections parallel with and at right 

 angles to the medullary rays. The same 

 applies to the ^opfl 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 (in similar direc- 

 tions) by the lapidary's wheel; wood in the 

 state of coal in like manner, or in the way 

 stated under Coal (see Preparation, 

 Fossil wood, and Coal). 



Gymnosperms. — In this division of the 

 Flowering Plants we also meet with two 

 types of structure. 



ConifercB. — Here the character of the wood 

 agrees in general with that of the typical 

 Dicotyledons, with certain distinctions ; 

 namely, although the medullary sheath of 

 spiral vessels exists, no ducts or vessels oc- 

 cur in the mass of wood external to this, 

 which is wholly composed of prosenchyma- 

 tous cells, with bordered pits, in single (PI. 

 39. fig. 6) (usually) or double or treble {Arau- 

 caria) rows (PI. 39. fig. 5) ; in Taxus accom- 

 panied in part by a spiral-fibrous band (PI. 

 39. fig. 6). The particulars of these forms 

 are given under Conifers. It may be 

 mentioned, that the ' woody parenchyma' of 

 Dicotyledons seems to be represented here 

 b}^ the cords of parenchymatous cells in some 

 cases traversing the prosenchyma, ultimately 

 filled with resinous deposits (" cords of se- 

 cretion cells "). 



Cycadacece. — The earliest condition of the 

 stems here appears to resemble that in Co- 

 niferae, but no annual rings are formed. 

 Concentric layers are produced at intervals, 

 however, separated by parenchymatous lay- 

 ers. The true mode of origin of these does 

 not appear to be clearly made out. The 

 wood is composed of pitted prosenchyma- 

 tous cells (PI. 39. fig. 20), without vessels or 

 ducts, excepting in the medullary sheath of 

 spiral vessels. 



For further details on the markings of the 

 ducts, &c., see Pitted and Spiral struc- 

 tures. 



The subject of the development of the 

 wood of stems has been more discussed per- 

 haps than any point in stiuctural botany. 

 We cannot enter u})on it here, beyond the 

 statement that the key to its comprehension 

 lies in the thorough appreciation of Schlei- 

 den's characters oi the Jibro-vascular bundles 

 in the different classes, and of the fact that 

 the cambium region exists at the growing 

 points and all over the outer surface of the 

 wood in Dicotyledons ; in a conical mass at 

 the summit alone, of Monocotyledons ; and 

 in a still more limited region at the summit 



