280 TISSUE ELEMENTS OF SEED PLANTS 



for thickening into the cell. The protoplasm generally 

 dies when the thickening is complete. The fibres are 

 commonly found in bands or masses not very far from 

 the surface of a herbaceous stem and in a similar position 

 in the midrib and larger veins of the leaf, but they 

 also form a large part of the wood of many woody 

 plants, for instance the oak, to which they give great 

 hardness and toughness. This depends partly on 

 changes in the cell wall (see below). Fibrous tissue 

 is used for making linen, canvas and paper, in which 

 the fibres are woven together to make the textile fabric. 

 The walls of the fibres in linen and the best paper are 

 nearly pure cellulose. 



Other thick-walled cells are box-shaped, spherical 

 or oval, and when the walls are very hard they are 

 called stone cells, which form the substance of the stone 

 in stone fruits (plum, cherry, etc.), and isolated cells 

 or nests of cells in the flesh of a gritty pear. Stone 

 cells frequently have beautiful branched pits (Fig. 44, C), 

 formed by the meeting and coalescence of several 

 different pits as the inner surface of the cell is diminished 

 by the progressive general thickening of the wall. 



Masses of thick-walled cells near the surface of an 

 organ may also have an important function in checking 

 loss of water by evaporation. 



The water-conducting cells of a plant are called 

 tracheids and vessels (Fig. 45). These are dead cells 

 with lignified walls (see below) , which form continuous 

 water-conducting channels throughout the plant parallel 

 with the sieve tubes A tracheid is usually an elongated 

 cell whose wall may be thickened in various ways. 



{a) Spiral tracheids have one or more lignified bands 

 of wall substance (Fig. 46) laid down on the inner 

 surface of the wall and running round and round the 



