FOEM AND AEKANGEMENT OF VESSELS. 



17 



In their ordinary form, Pleurenchymatous tubes have no definite 

 markings on their walls ; but in some instances markings present 

 themselves in the form of simple discs (fig. 48), 

 or of discs with smaller circles in the centre 

 (fig. 49). These discs occur in the wood of Firs, 

 Pines, and Winter's bark, which has received 

 the name of glandular or punctated woody tissue. 

 The markings are formed by concave depres- 

 sions on the outside of the walls of contiguous 

 tubes, which are closely applied to each other, 

 forming lenticular cavities between the vessels, 

 like two watch-glasses in apposition, and when 

 viewed by transmilited light they appear like 

 discs (fig. 48). 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. 49). 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. 

 50). That these markings are cavities between the 

 fibres was prov.ed by Quekett in the case of fossil pine 

 wood, where he separated lenticular masses of solid matter 

 from the discs. There is sometimes observed a thicken- 

 ing layer, in the form of a spiral fibre, surrounding the 

 discs more or less completely, as in the yew. The discs 

 are usually arranged in single rOws, but they occur also 

 in double and triple rows, as in Araucaria, where the 

 markings alternate with each other. 



Pibeo-Vasculak Tissue, or Trachenchynia (trachea, 

 windpipe ; f^oi.yvs, rough), is formed of membranous 

 tubes tapering at each end, less firm than Pleurenchyma, 

 and either having a fibre coiled up spirally in their in- 

 terior, or having the membrane marked with rings, bars, 

 or dots, arranged in a more or less spiral form. 



TuuE Spiral Vessels (spiroidea, trachece), constituting 

 the typical form, present themselves as elongated tubes 

 clustered together, overlapping each other at their conical 

 extremities, and having a spiral fibre or fibres surrounding 

 the interior of the cylinder (fig. 51). Their outer mem- 

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



Kg. 48. Woody tubes, with circular spots where the memhrane is thin, Bigrumia. Pig. 49. 

 Punctated woody tissue, with double circles or discs, froin common Scotch fir. Pig. 60. Lon- 

 gitudinal section of the same, showingthe union between the fibres, and the mode in which the 

 circles are formed. Pig. 61. Two spiral vessels united. Pig. 62. Simple trachea, with fibre 

 uncoiled. Pig. 53. Spiral vessel with a ribband of united fibres (Pieio(racftea),from the Banana, 







Fig. 52. 



Figs. 



