SPIRAL VESSELS. 



373 



FIG 161. 



tnte of vessels, they afford the sole channel for the ascent of the 

 sap. I^ut after their walls have become thickened by internal 

 deposit, they are no longer subservient to this function ; nor, in- 

 deed, do they 'then appear to fulfil any other purpose in the 

 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 ( 238). A peculiar set of markings, 

 seen on the woody fibres of the Coniferce, 

 and of some other tribes, is represented in 

 Fig. 161 ; 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 indi- 

 cates 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 sometimes 

 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 con- 

 stancy of their arrangement, that they 

 fulfil some important object in the eco- section of coniferous wood 

 nomy of the plants in which they occur; the direction of the fibres, showing 

 and there are varieties in this arrangement 

 so characteristic of different tribes, that it 

 is sometimes possible to determine, by the microscopic inspec- 

 tion 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." 



232. 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" ( 228), 

 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 charac- 

 ter 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") surround- 

 ing 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 ; but their structure may be more easily dis- 



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 anymore 

 claim to the designation of vessels, than have the elongated cells of the ligneous tissue. 



