FORM AND ARRANGEMENT OF VESSELS. 



pear like discs (fig. 46). In the centre of the depression there is a 

 canal, often funnel-shaped, and the part of the tube corresponding to it 

 being thus thinner than the surrounding texture, gives the aspect of a 

 smaller circle in the centre (fig. 47). When a thin section is made 

 through two parallel lines of punctations, the slits or fissures are seen 

 which give rise to the appearances mentioned (fig. 48). That these mark- 

 ings are cavities between the fibres was proved by Quekett 

 in the case of fossil pine wood, where he separated lenticu- 

 lar masses of solid matter from the discs. There is some- 

 times observed a thickening layer, in the form of a spiral 

 fibre, surrounding the discs more or less completely. The 

 discs are usually arranged in single rows, but they occur 

 also in double and triple rows, more particularly in Arau- 

 caria and Altingia. 



31. Fibre- Vascular Tissue, or Trachenchyma (trachea, 

 windpipe ; rpet^vg, rough), is formed of membranous tubes 

 tapering at each end, less firm than Pleurenchyma, and 

 either having a fibre coiled up spirally in their interior, or 1 

 having the membrane marked with rings, bars, or dots, I 

 arranged in a more or less spiral form. 



32. True Spiral Teasel* (spiroidea, trachece), constituting | 

 the typical form, present themselves as elongated tubes | 

 clustered together, overlapping each other at their conical j 

 extremities, and having a spiral fibre or fibres surrounding ; 

 the interior of the cy Under (fig. 49). Their outer mem- 

 brane is thin, and consists of pure cellulose. At the point \ 

 where they overlap, it is sometimes absorbed so as to i 

 allow direct communication between the vessels. The 

 fibre or spiral filament is generally single, forming simple 49 

 trachece (fig. 50); but sometimes numerous fibres, varying from two to 

 more than twenty, are united together, assuming the aspect of a broad 

 ribband (fig. 51), and constituting Pleiotrachece (^M'IUI>, more). The 

 fibre is elastic, and can be unrolled. This can be seen by taking the 

 leaf of a Pelargonium, and after making a superficial cut round the 

 stalk, pulling the parts gently asunder, when the fibres will appear like 

 the threads of a cobweb. 



33'. Spiral vessels were first noticed as early as 1661, by Henshaw. 

 They occur principally in the higher classes of plants, and are well seen 

 in annual shoots, as in Asparagus; in the stems of Bananas and Plan- 

 tains, where the fibres may be pulled out in handfuls, and used as tin- 

 der ; in many aquatics, as Nelumbium and Nymphasa, and in Liliaceous 

 plants. In hard woody stems, they are principally found in the sheath 



Fig. 49. Two spiral vessels united. 



Fig. 50. Simple trachea. 



Fig. 51. Spiral vessel with a ribband of united fibres (Pleiotrachea,) from the Banana. 



