14 



FORM AND ARRANGEMENT OF VESSELS. 



have called this tissue Prosenchyma (K&, close to, in reference to the 

 close apposition of the tubes), a term, however, generally 

 applied to shortened fusiform cells only. Pleurenchyma- 

 tous vessels lie close together, overlap each other, and, by 

 their union, give strength and solidity to the plant. Their 

 membrane becomes thickened by successive deposits of 

 layers of cellulose and sclerogen, and in a transverse sec- 

 tion the tubes present tHe appearance of concentric circles, 

 occasionally with intervals, where the ligneous matter is 

 deficient (fig. 45). The wood of trees is made up of fibres 

 or tubes of this kind, and they are found in the inner 

 bark, and in the veins of leaves. The woody fibres may 

 be separated from the cellular parts of plants by macera- 

 tion. In this way Flax and Hemp are procured, as Avell 

 as the Bast used for mats. The strength of the woody 

 fibres of different plants varies. Thus, New Zealand Flax, 

 the produce of Phormium tenax, is superior in tenacity to 

 Common Hemp; while the latter, in its turn, excels Com- 

 mon Flax, as well as Pita Flax, which is the produce of 

 Agave americana. Linen is formed from woody tissue. 

 Cotton, on the other hand, consists of elongated cells or 

 hairs, the membrane of which becomes contracted in the 

 process of drying, so as to appear twisted when viewed 

 under the microscope. By this character mummy cloth 

 was shown to be composed of linen. Woody fibres, in 

 fabric, form muslin, lace, &c., some fine India muslins 

 only are formed from woody fibre ; other muslins are 

 made of cotton ; when reduced to small fragments, they constitute the 

 pulp whence paper is made. 



30. In its ordinary form, Pleurenchyma has no definite markings 

 on its walls; but in some instances these pre- 

 sent themselves in the form of simple discs (fig. 

 46), or of discs with smaller circles in the 

 centre (fig. 47). The latter occurs in the wood 

 of Firs, Pines, and "Winter's bark, and has re- 

 ceived the name of glandular or punctated woody 

 tissue. These markings are formed by concave 

 depressions on the outside of the Avails of .conti- 

 guous tubes, which are closely applied to each 

 other, forming lenticular cavities between the 

 vessels, like two watch-glasses in apposition, 

 and when viewed by transmitted light they ap- 



Fig. 44. Woody fibres (Pleurendiyma,) from Clematis vitalba. 

 Fig. 45. Transverse section of the same. 



Fig. 46. Woody fibres with circular spots where the membrane is thin (Rignonia). 

 Fig. 47. Punctated woody tissue, with a double circle or disc, from common Scotch fir. 

 Fig. 48. Longitudinal section of the same, showing the union between the fibres and the mode 

 in which the circles are formed. 



