FOEMATION OF WOOD 273 



the wood and bast. The protoxylem or first 

 formed wood is seen to be composed of a number 

 of elongated cells which have lost their proto- 

 plasmic contents, have become lignified as to 

 their walls, the thickening taking the form of 

 annular or spiral bands, much as a reinforced 

 hose-pipe is strengthened by a wrapping of 

 stout wire, with this difference, that in the case 

 of the hose-pipe the thickening or strengthening 

 is external whilst in the wood cells it is internal. 

 These thickened cells may be tracheids or 

 tracheae, which may be distinguished by the 

 fact that the latter are formed by the fusion of 

 vertical rows of cells and the former are not so 

 formed. The protoxylem of conifers consists of 

 tracheids and tracheae, that of the broad-leaved 

 trees consists of tracheae. In the latter trees 

 the secondary xylem or later formed wood is . 

 of a somewhat complex nature, and we must 

 refer the reader to a textbook of botany for 

 precise details. SuflB.ce it to say that this 

 secondary wood may comprise tracheids, 

 tracheae, wood fibres, fibrous cells with either 

 thick or thin walls and wood parenchyma ; the 

 one kind of cells which are always present are 

 the tracheae or true vessels. 



In longitudinal section, the bast is seen to be 

 composed of hard and soft bast ; the hard bast 

 again is composed of bast fibres, whereas three 



