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A TEXTBOOK OF THEORETICAL BOTANY 



Among the arboreal Dicotyledons (which includes most of our forest 

 trees) the end walls of the elements are very oblique and are partly lignified, 

 so that the formation of vessels involves only the solution of the pit membranes 

 between the bars of lignin. In herbaceous plants, however, the vessels are 

 usually broader and their elements become united before their differentiation 

 is complete. The cell elements involved are often very short and the end 

 walls transverse and unthickened, so that they disappear completely, leaving 

 at most a trace of the rim (Fig. 454). This usually happens before the 

 protoplasm of the individual cells has disintegrated. 



A B 



Fig. 454. — -Comparison of terminal perforations in vessel elements. A, Helianthus 

 annuus. Open pore. B, Salix caprea. Scalariform end walls. 



Vessels show the same types of differentiation in wall thickening that 

 are found in tracheids. In addition, however, large vessels show a coarse 

 network of markings, due to contacts with the walls of smaller adjacent 

 tracheids. 



Vessels are not of indefinite length. In certain trees they may be many 

 feet long, perhaps even the whole length of the trunk, but the average length 

 is much less, probably less than 3 ft. Usually the vessels are considerably 

 wider than other elements of the wood, though exceptions occur, as in the 

 very fine-grained wood oi Buxus (Box), which gets its name from the German 

 Bucks, on account of its use in making wood-cuts for illustrations. Climbers, 

 especially the woody climbers or lianas of the tropics, have notably large 

 vessels, sometimes more than i mm. across, which is associated with the great 

 length of their stems. 



