THE TISSUES OF PLANTS. 



33 



vessels are thickened in such a way as to give rise to pits 

 and dots. The vessels are usually of wide diameter ; in 

 some forms they are crossed at frequent intervals by per- 

 forated horizontal or inclined septa (Fig. 22); in other 

 forms they have fusiform extremities. 



54. (4) Trache'ids. — These consist for the most part of 

 single closed cells; otherwise they possess the characters 

 of vessels. In one form (Fig. 33), as in the so-called wood- 



FiQ. 23. Fig. 24. 



Fig. 23. — Ends of several trachelds from the wood of a Pine, showing 

 bordered pits. Magnified 325 times. ... 



Fig. 24.— Trachelds from the stem of Laburnum, m, to, cells of a medul- 

 lary ray. At g, a partition is broken through. Magnified 375 times. 



cells of Conifers, they are intermediate in structure be- 

 tween the pitted'ressels and the fibres of the wood of other 



