INTRODUCTION TO PLANT HISTOLOGY 



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a dome of the lignified secondary wall. Each dome has a small opening at 

 its apex giving entrance to the pit cavity. The pit membranes are very thin 

 and are indeed sometimes perforate, so that fluids move very easily from 

 tracheid to tracheid, even in these mature types. Frequently a small thickened 

 disc, the torus, is formed in the centre of the membrane and in the highest 

 type of bordered pit, as in Pimis, the membrane can be displaced from side 

 to side, the torus exactly filling the opening in the dome, so that a valve 

 action is secured. Bordered pits are characteristic of dead cells, such as 

 tracheids, where there is no protoplasm to support the pit membrane. 



One type of tracheid has narrow, transverse pits, like slits. This is called 

 scalariform, from its likeness to the rungs of a ladder. It is the characteristic 

 type in the Pteridophyta, but it also occurs in other groups. 



Vessels 



Tracheids are individual cells, but where they are in contact end to end, 

 the thin end walls may disappear, so that a chain of tracheids becomes an 



B 



Fig. 453. — Ulmus procera. Comparison of tracheid and vessel element. 

 A, Pitted tracheid. B, Pitted vessel element showing sub-terminal 

 pores. Isolated by maceration. 



open tube, called a vessel or trachea (from its resemblance to the tracheae of 

 insects) (Fig. 453). Vessels are almost confined to the Angiosperms, where 

 they make up an important part of the wood. If they are well developed they 

 practically supersede the tracheids for the conduction of water, and the latter 

 then function simply as water stores, and may be limited in number to a few 

 around the vessels. The cellular units which make up a vessel are called 

 the vessel elements. 

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