MECHANICAL OR SUPPORTING CELLS 279 



one wall only may be thickened. The external walls 

 of the epidermal cells covering the shoot are generally 

 considerably thicker than the lateral and inner walls, 

 and sometimes the outer wall is very thick in the centre 

 and lens-shaped (Fig. 43, E). In collenchyma, the sup- 

 porting tissue of elongated living cells commonly met 

 with just below the epidermis in herbaceous stems and 

 in the midribs of leaves, the thickening is confined to 

 the corners of the cells, so that longitudinally running 

 pillars of cellulose are formed, connected with the 

 neighbouring pillars by thin membranes (Fig. 44, A, B). 

 Thus there is a certain amount of " give " under 

 horizontal strain in this tissue. In many cells ribs or 

 other local projections of various shapes are laid down 

 on the inner surface of the wall. The spiral thickenings 

 on the walls of the elaters of Pellia, and various similar 

 thickenings in the cells of Seed Plants (Fig. 45, A-G), 

 are cases in point. 



Dead Cells. In many cells of the plant body the 

 protoplasm dies after the development of the cell 

 is completed, the wall alone remaining. While these 

 dead cells can no longer take an active part in the life 

 of the plant, they often form an integral part of its 

 structure, and many of them are of essential importance. 

 The two chief functions they carry out are those of 

 mechanically strengthening the plant body and of 

 water conduction. The chief mechanical or supporting 

 tissues are composed of thick-walled elongated cells 

 with long tapering ends, which fit between one another, 

 so that the tissue is very tough and not easily broken 

 by strains. Such cells are called fibres (Fig. 44, D, E). 

 The wall of the fibre is sometimes very thick, almost 

 obliterating the cell cavity. It is usually penetrated 

 by narrow pits, which serve to bring the sugars used 



