364- THE MICROSCOPE AND ITS REVELATIONS. 



spindle-shaped, whose walls have a special tendency to undergo consoli- 

 dation by the internal deposit of sclerogen. It is obvious that a tissue 

 consisting of elongated cells, adherent together by their entire length, 

 and strengthened, by internal deposit, must possess much greater tenacity 

 than any tissue in which the cells depart but little from the primitive 

 spherical form-; and we accordingly find Woody fibre present wherever it 

 is requisite that the fabric should posses not merely density, but the 

 power of resistance to tension. In the higher classes of the Vegetable 

 Kingdom it constitutes the chief part of the stem and branches, where 

 these have a firm and durable character; and even in more temporary 

 structures, such as the herbaceous stems of annual Plants, and the leaves 

 and flowers of almost every tribe, this tissue forms a more or less impor- 

 tant constituent, being especially found in the neighborhood of the spiral 

 vessels and ducts, to which it affords protection and support. Hence the 

 bundles of fasciculi composed of these elements, which form the e veins' 

 of leaves, and which give ( stringiness ' to various esculent vegetable sub- 

 stances, are commonly known under the name of fibro-vascular tissue. 

 In their young and unconsolidated state, the ligneous cells seem to con- 

 duct fluids with great facility in the direction of their length; and in the 

 Coniferous tribe, whose stems and branches are destitute of ducts, they 

 afford the sole channel for the ascent ot the sap. But after their walls 

 have become thickened by internal deposit, they are no longer subservient 

 to this function: nor, indeed, do they then appear to fulfil any other 

 purpose in the Vegetable economy than that of affording mechanical sup- 

 port. It is this which constitutes the difference between the alburnum or 

 ' sap- wood/ and the duramen or 'heartwood,' of Exogenous stems 

 ( 369). 



361. A peculiar set of markings seen on the Woody fibres of the Con- 

 ifer CB, and of some other tribes, is represented in Fig. 248; in each of 

 these spots the inner circle appears to mark a deficiency of the lining 

 deposit, as in the pitted cells of other plants: whilst the outer circle 

 indicates the boundary of a lenticular cavity which intervenes between 

 the adjacent cells at this point. There are varieties in this arrangement 

 so characteristic of different tribes, that it is sometimes possible to 

 determine, by the ^microscopic 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. 



362. All the more perfect forms of Phanerogamia contain, in some 

 part of their fabric, the peculiar structures which are known as spiral 

 vessels.' 1 These have the elongated shape of woody fibres; but the 

 internal deposit, as in the spiral cells ( 357), takes the form of a spiral 

 fibre winding from end to end, remaining 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 elon- 

 gated fibre-cells of the Nepenthes (Chinese Pitcher-plant). Such cells 

 are especially found in the delicate membrane (medullary sheath) sur- 

 rounding the pith of Exogens, and in the midst of the woody bundles 

 occurring in the stem of Endogens; thence they proceed in each case to 

 the leaf-stalks, through which they are distributed to the leaves. By 

 careful dissection under the Microscope, they may be separated entire; 



1 So long, however, as they retain their original cellular character, and do not 

 coalesce with each other, these fusiform spiral cells cannot be regarded as having 

 any more claim to the designation of vessels, than have the elongated cells of the 

 ligneous tissue. 



