402 



WOODY FIBRE. SPIRAL VESSELS. 



Fig. 210. 



Vegetable economy than that of affording Mechanical support. It 

 is this which constitutes the difference between the Alburnum or 

 'Sap-wood,' and the Duramen or 'Heart-wood,' of Exogenous 

 Stems (§ 301). — A peculiar set of markings seen on the Woody- 

 fibres of the Coniferce, and of some other tribes, is represented 

 in Fig. 210 : in each of these spots the inner circle appears to 



mark a deficiency of the lining 

 deposit, as in the porous cells of 

 other plants ; whilst the outer 

 circle indicates the boundary of a 

 lenticular cavity which intervenes 

 between the adjacent cells at this 

 point, and which contains a small 

 globular body that may be some- 

 times detached. Of the purpose 

 of these minute bodies interposed 

 between the Wood- Cells, nothing 

 is known ; there can be no doubt, 

 however, from the definiteness and 

 constancy of their arrangement, 

 that they fulfil some important 

 object in the economy of the 

 Plants in which they occur ; and 

 there are varieties in this arrange- 

 ment so characteristic of differ- 

 ent Tribes, that it is sometimes 

 possible to determine, by the mi- 

 croscopic inspection of a minute 

 fragment, even of a Fossil wood, 

 the tribe to which it belonged. 

 The Woody Fibre thus marked is 

 often designated as glandular. 



295. All the more perfect forms 

 of Phanerogamia contain, in some 

 part of their fabi-ic, the peculiar structures which are known 

 as Spiral vessels.* These have the elongated shape of Woody 

 Fibres ; but the internal deposit, as in the Spiral Cells (§ 291), 

 takes the form of a spiral fibre winding from end to end, remain- 

 ing distinct from the cell-wall, and retaining its elasticity ; this 

 fibre may be single, double, or even quadruple — this last character 

 presenting itself in the very large elongated Fibre-Cells of the 

 Nepenthes (Chinese Pitcher-plant). These cells are especially 

 found in the delicate membrane (Medullary Sheath) surrounding 

 the Pith of Exogens, and in the midst of the woody bundles 



* So long, however, as they retain their original cellular character and 

 do not coalesce with each other, these fusiform spiral cells cannot be re- 

 garded as having any more claim to the designation of vessels, than have 

 the elongated cells of the ligneous tissue. 



Section of Coniferous Wood 

 in the direction of the Fibres, 

 showing their ' glandular ' 

 dots : — a a a, Medullary Rays 

 crossing the fibres. 



