48 THE ELEMENTARY STRUCTURE OF PLANTS. 



eye like a fine cobewb. In stems furnished with pith, the spiral 

 vessels usually occupy a circle immediately around it. They occur 

 also in the veins of the leaves, and in all parts which are modifi- 

 cations of leaves. More commonly the coil is formed of a single 

 fibre, as in Fig. 62, 63 : it rarely consists of two fibres ; but not 

 uncommonly of a considerable number, forming a band, as in Fig. 

 64. Spiral vessels of the latter kind are to be found in an Aspara- 

 gus shoot, and are finely seen in the stems of the Banana. From 

 the Musa textilis of Manilla, of the same genus as the Banana, 

 these cobwebby fibres are said to be extracted in large quantities, 

 and used in the production of the most delicate of textile fabrics. 



61. True spiral vessels, capable of uncoiling, occur in all plants 

 of the higher grades, but only in particular parts. Reticulated and 

 annular ducts abound in most herbaceous stems ; and every transi- 

 tion may be detected between the various kinds. Fig. 65 shows a 

 number of variations, such as may be seen at one view in the stem 

 of a Polygonum. Some have the fibre closely coiled ; in others the 

 turns are distant. Some are simple tubes, and apparently formed of 

 a single elongated cell : others show cross partitions, or vestiges of 

 them, and so are made up of a row of cells ; and if these be com- 

 pared with Fig. 39 - 43, &c, it will plainly appear that ducts of all 

 sorts are only a modification of ordinary cells. Even the longest 

 are of no great length ; very rarely are they above half an inch 

 long ; and they terminate by closed ends, like all other cells ; the 

 termination being either abrupt or more commonly conical or ob- 

 tusely pointed. In young parts the ducts, like other cells, contain 

 liquid, the ordinary juices of the plant: in older stems they are 

 filled with air, except when the whole tissue is gorged with sap, 

 which then finds its way into these also. 



62. Interlaced Fibrilliform Tissue. This is quite as distinct from 



ordinary cellular tissue, and as worthy of a special name, as any of 

 the kinds already described. It is the more worthy of notice, 

 from its near resemblance to some forms of animal tissue. It 

 consists of very long, much attenuated, simple or branching, fibre- 

 like cells, or strings of cells, inextricably entangled or interwoven 

 without order, so as to make up a loose, fibrous tissue. It is prin- 

 cipally met with in Fungi, Moulds, &c, where the cells are ex- . 

 tremely soft and destructible ; and in Lichens (Fig. 25), where it is 

 dry and much firmer. A remaining and a very ambiguous element 

 of vegetable fabric is 



