WOOD OF BROAD-LEAVED TREES. 33 



of wood and being sometimes specially conspicuous in the spring- 

 wood. Some of them, in young wood, have net-like thickening, 

 but most of them have bordered pits, as have also the tracheids. 

 The chief differences in fact between these two kinds of elements 

 are the smaller diameter and lesser length of the tracheids. As 

 they are each formed from a single cambium cell, these tracheids 

 have no transverse divisions ; whereas in the vessels there are 

 much-perforated or partially absorbed 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 tracheaB, and are often more pointed, but their distinctive 

 characteristic is their much-thickened, lignified walls, marked 

 with few simple pits, often oblique and narrow. This thickening 

 of their walls sometimes almost obliterates 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 trans- 

 verse walls. Wood parenchyma consists of vertical groups of 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 lignified and pitted, having bordered pits 

 where in contact with tracheaB or tracheids, but simple pits 

 elsewhere. Wood-parenchyma is commonly grouped in narrow 

 circles round the vessels, appearing in longitudinal sections as 

 cloudy margins to them. It may expand from such circles 

 laterally into wings forming 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 ("false rings"). These transverse lines of tissue 

 may be very narrow, as in Ebonies, or broad and conspicuous. 

 Wood-parenchyma much resembles the pith-rays, especially in 



C 



