16 



FORM AND AEEANGEMENT OF VESSELS. 



Section II. — ^Vasculah Tissue. 



1. Form and Arrangement of Vessels. 



Vasculae Tissue, or Angienchyma (ayyog, a vessel), consists of 

 tubes, whose length greatly exceeds their breadth. These may be 

 formed of membrane only, or of membrane altered in various ways by 

 deposits of fibre, or of thickening matter. 



Fibrous Tubes, or Ligneous Tissue, Pleurenchyma ('irXivgd, a 

 rib, from its firmness), (fig. 46), consists of tubes, or, according to 

 some, elongated cells, of a fusiform (fusus, a spindle) or spindle-like 

 shape (fig. 3), having their walls thickened so as to give great firm- 

 ness. This form of tissue does not exist in cellular plants. Some 

 have called this tissue Prosmchyma, a term, however, generally ap- 

 plied to shortened fusiform cells only. Pleurenchyma- 

 tous vessels lie close together, overlap each other, an3, 

 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 

 section the tubes present the appearance of concentric 

 circles, occasionally with intervals, where the ligneous 

 matter is deficient (fig. 47). 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 fibrous 

 tissue may be separated from the cellular parts of plants 

 by maceration. In this way Flax and Hemp are pro- 

 cured, as well as the Bast used for mats. The strength 

 of the fibres of different plants varies. Thus, New Zear 

 land Max, the produce of Phormium tenax, is superior 

 in tenacity to Common Hemp ; while the latter, in its 

 turn, excels Common 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 be- 

 comes contracted in the process of drying, so as to appear 

 twisted when viewed under the microscope. By this cha- 

 racter mummy cloth was shown to be composed of 

 linen. Fibrous tissue, in fabric, forms muslin, lace, etc. 

 (some fine Indian muslins only are formed from this 

 tissue ; other muslins are made of cotton) ; when 

 reduced to small fragments they constitute the pulp 



I 



Kg. 46. 



Kg. 47. 



whence paper is made. 



Kg. 46. Fibres of Pleurenchyma, from Clematis Vitalba. 

 of the same. 



Kg. 47. Transverse section 



