318 



MANUAL OF BOTANY 



thickened in the same way as the lateral ones (fig. 683). The 

 individual cells are then known as trache'ids. 



Tracheids occur in other forms than as the cells of a column ; 

 thus they constitute a somewhat parenchymatous-looking tissue 

 in the sheath of the aerial roots of certain orchids. They are 

 always lignifled and usually pitted. There is no very sharp 

 distinction between a fibre and a tracheid when the latter does 

 not appear as a segment of a vessel. Thus the peculiar fibres 

 with bordered pits, occurring in the secondary wood of the Coni- 

 ferce, are generally included under this term. 



The chief forms of tracheal tissue, whether composed of 

 tracheids or true vessels, may now be separately considered. 



a. Pitted or Dotted Vessels. — A pitted vessel is formed from 



I"IG. 684. 



Fig. 685. 



Fig. 686. 



Fig. 687. 



Fig. 684. Simple spiral vessels. Fig. 685. Compound spiral vessel. 



Fig,Q8Q. Branched spiral vessel. Fig. 687. Union of spiral vessels in 



au oblique manner. 



a row of cylindrical pitted cells placed end to end (fig. 682), by 

 the absorption of the separating walls. This mode of origin is 

 clearly shown in many instances by the constrictions which its 

 sides exhibit at various intervals {fig. 682) ; for these constric- 

 tions evidently correspond to the points where the component 

 cells come in contact. In many cases, indeed, both in these and 

 the spiral, annular, reticulated, and scalariform vessels about to be 

 described, the transverse partitions are not absorbed, and columns 

 of tracheids are formed instead of true vessels. Pitted vessels may 

 be commonly foundinthe wood of Dicotyledons ; they are mixed 

 there with the ordinary wood-cells, but are much larger than 

 these ; indeed, they are generally among the largest occurring 

 in any tissue. 



It sometimes happens that when a pitted or other vessel has 



