2S 



OF WOOD IN GENERAL 



whereas in the vessels there are much-perforated or partially ab- 

 sorbed partitions inclined towards the pith-rays, indicating the 

 origin of the vessels from the fusion of a chain of cells. Woody 

 fibres may be as long as, or longer than, the tracheae, and are often 

 more pointed, but their distinctive characteristic is their much- 

 thickened, hgnified walls, marked with few simple pits, often oblique 

 and narrow. This thickening of their walls sometimes almost 

 obhterates the cell-cavity or lumen, and, together with their early 

 loss of all contents but water and air, serves to indicate their main 

 function to be that of mechanical support. Fibrous cells only differ 

 from fibres in retaining their protoplasmic contents. Their walls 

 sometimes remain thin. Both thick-walled fibrous cells and woody 

 fibres sometimes become chambered by the formation of delicate 

 transverse walls. Wood-parenchi/ma consists of vertical groups of 



Pio 24.— Two annual nngs of wood and the bark of the Oak, the upper surface m 

 transverse section, part of the inner nng (unshaded) m tangential, and the front 

 •new of "both nngs m radial section The medullary rays are shown black in trans» 

 veise, shaded m radial section, (After Hough ) 



short cells, the upper and lower cell of each group tapering to a 

 point, each group originating, in fact, from the transverse division 

 of one cambium-cell. They retain their protoplasm and become 

 filled with starch in autumn. Their walls are not much thickened, 

 but are hgnified and pitted, having bordered pits where in contact 

 with tracheae or tracheids, but simple p^ts elsewhere. Wood- 

 parenchyma is commonly grouped in narrow circles round the 

 vessels, appearmg in longitudinal sections as cloudy margins to 

 them. It may expand from such circles laterally into wings form- 

 ing a spindle-shaped patch with the vessel in the centre, and these 

 wings may widen until they meet others, so forming straggling 

 oblique lines, long wavy streaks, or concentric circles (''fake 

 rings''). These transverse Knes of tissue may be very narrow^ 

 as in Ebonies, or broad and conspicuous. Wood-parenchyma much 



