24 BOTANY. 
The vessels are usually of wide diameter; in some forms 
they are crossed at frequent intervals by perforated hori- 
zontal or inclined septa (Fig. 17); in other 
forms they have fusiform extremities. | 
52. (4) Tracheids—These consist for 
the most part of single closed cells; other- 
wise they possess the characters of vessels. 
In one form (Fig. 18), as in the so-called 
wood-cells of Conifers, they are interme- 
diate in structure between the pitted ves- 
sels and the fibres of the wood of other 
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Fie. 16. Fie. 17. 
Fic. 16.—Scalariform vessels of the common Brake (Pteris). 
Fie. 17.—Pitted vessels of Dutchman’s Pipe (Aristolochia sipho), from a longi- 
tudinal section of the stem; the vessel on the right is seen in section, that on 
the left from without. aa, rings. which are remnants of the original trans- 
verse partitions; b, b, sections of the walls. 
flowering plants. Every gradation between these tracheids 
and the other forms of tracheary tissue occur. In another 
form, as in the wood of many common trees and shrubs, 
the tracheids are shorter than in the preceding, quite 
