MORPHOLOGY 



137 



of water conduction. In the wood strands of these plants there are also present 

 wood parenchyma and a large proportion of wood fibres. The vessels in climbing 

 plants (lianas) are especially wide. 



The distribution of the living elements in the wood strands always bears a dis- 

 tinct relation to the water-courses which they accompany, enclosing them in a more 

 or less complete sheath. The living cells adjoining the tracheal elements are in 

 communication with them by means of one-sided bordered pits. When such pits 





ft 



Fio. 145. .4, Elements of the tracheal tissue of the wood ; diagrammatic. /.', Elements of the 

 parenchymatous tissue of the wood ; diagrammatic. For description see text. 



occur between living cells and tracheal (elements the pit cavities are absent on the 

 side of the living cell, but present in the tracheal elements ; they differ from the 

 true bordered pits in the absence of a torus on the pit membrane. 



The Elements of the Secondary Phloem in Gymnosperms and 

 woody Dicotyledons are sieve-tubes, or these together with companion 

 cells, bast parenchyma with abundant cell contents, and long narrow 

 bast fibres with strongly thickened walls. The sieve-tubes serve to 

 conduct proteid materials ; the companion cells, or in their absence 

 special rows of the bast parenchyma, take up substances from the 



