SECT. i. LIGNEOUS TISSUE. 175 



tissues which form the wood do not convey the rising 

 sap in equal quantity. The outermost layers that are 

 nearest the bark, which are always the last formed or 

 youngest, convey it in greatest quantity, and on that 

 account are called the sap-wood : the older the layers 

 the less they convey, because the interior walls of the 

 cells of both tissues are coated with successive layers of 

 a mucilaginous substance which is the colouring matter 

 of the wood (lignin), and is called sclerogen, which be- 

 comes hard, is ultimately united to the cell walls, and 

 fills or nearly fills the tubular fibres and vascular ducts, 

 so that those nearest the centre of the tree lose or nearly 

 lose the power of conducting the sap, as in hard wood 

 like the oak, though in softer wood, as the lime tree, it is 

 not entirely lost. Ligneous tissue forms the chief part 

 of the stems, branches, and shoots of trees and shrubs ; 

 it gives firmness to leaves, flowers, and all their parts, 

 and strength to the stems and skins of herbaceous plants ; 

 it is found in the bark of all trees, and constitutes the 

 strong fibre of hemp, flax, the agave, and many other 

 plants, whence linen, canvas, and cordage are made. 

 Cells lined with sclerogen form the shells of nuts, cocoa- 

 nuts, and walnuts, as well as cherry, peach, and plum 

 stones, the brown coat of apple and pear seeds, the gritty 

 particles in the heart of the pear, the white coats of the 

 pips of the orange and lemon, the husks of peas, &c. 



All the tissues are represented in fig. 5, which is a 

 longitudinal section of the Italian reed, much magnified. 

 It consists of three parts : at a the cellular tissue of the 

 pith is represented ; 6 is a fibro- vascular bundle contain- 

 ing annular ducts (1), spiral ducts (2), a pitted duct (3), 

 besides the long spindle-shaped threads of woody fibre ; 

 c is the exterior part of the reed, which consists of cellular 

 tissue, the two surface rows being rather compressed 

 and filled with coloured particles. 



Besides the spiral vessels that are attached to the 



