698 MICROSCOPIC STRUCTURE OF PHANEROGAMIC PLANTS 



order, are known as bordered pits, and the elongated cells in which 

 they occur as tradte'ids. 



All the more perfect forms of Phanerogams contain, in some 

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

 spiral vessels. 1 These have the elongated shape of fibre-cells ; but 

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

 fibre winding from end to end, and retaining its elasticity ; this 

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

 senting itself in the very large elongated fibre-cells of Nepenthes 

 (pitcher-plant). Such vessels are especially found in the delicate 

 membrane (medullary sheath) surrounding the pith of Exogens, 

 and in the ' xylem-portion ' of the woody bundles of Exogens and 

 Endogens ; thence they, proceed to the leaf -stalks, through which 

 they are distributed to the leaves. By careful dissection under the 

 microscope these fibro- vascular bundles may be separated entire ; 

 but their structure may be more easily displayed by cutting round, 

 but not through, the leaf-stalk of the strawberry, geranium, &c., and 

 then drawing the parts asunder. The membrane composing the 

 tubes of the vessels will thus be broken across ; but the fibres within, 

 being elastic, will be drawn out and unrolled. Spiral vessels are 

 sometimes found to convey fluid, whilst in other cases they contain 

 air only. 



Although fluid generally finds its way with tolerable facility 

 through the various forms of cellular tissue, especially in the direction 

 of the greatest length of the cells, a more direct means of connection 

 between distant parts is required for its active transmission. This is 

 afforded by the peculiar kind of vessels known as ducts, which consist 

 of cells laid end to end, the partitions between them being more or 

 less obliterated. The origin of these ducts is occasionally very evi- 

 dent, both in the contraction of their diameter at regular intervals, 

 and in the persistence of remains of their partitions (fig. 551, 

 b, b) ; but in most cases it can only be ascertained by studying the 

 history of their development, neither of these indications being trace- 

 able. Some of these ducts (fig. 537, 2) are indistinguishable from 

 the spiral vessels already described, save in the want of elasticity in 

 their spiral fibre, which causes it to break when the attempt is made 

 to draw it out. This rupture would seem to have taken place, in 

 some instances, from the natural elongation of the cells by growth, 

 the fibre being broken up into rings, which lie sometimes close 

 together, but more commonly at considerable intervals ; such a duct 

 is said to be annular (fig. 537, i). Intermediate forms between the 

 spiral* and annular ducts, which show the derivation of the latter 

 from the former, are very frequently to be met with. The spirals are 

 sometimes broken up still more completely, and the fragments of the 

 fibre extend in various directions, so as to meet and form an irregular 

 network lining the duct, which is then said to be reticulated. The 

 continuance of the deposit, however, gradually contracts the meshes, 



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 

 woody tissue. 



