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A TEXTBOOK OF THEORETICAL BOTANY 



continuous passage. This involves the akeration or disappearance of the 

 transverse walls which separate the young elements from each other. 



Comparative studies have shown that in the wood of certain families 

 which are predominantly arboreal in habit the vessel elements are narrow 

 and long and have steeply oblique end walls wdth numerous scalariform 

 perforations (Fig. 880). In this they resemble the vessels of the Gnetales. 

 Examples are : Magnolia, Betiila, Salix, Myrica. From this unspecialized 

 condition there has been a progression towards vessels with short elements, 

 as broad as or broader than their length, and with transverse end walls which 

 have either one wery large central perforation, or are, in the final stage, 



A B 



Fig. 880. — Comparison of terminal perforations in vessel elements. 

 A, Helianthus annuus. Open pore. B, Salix caprea. Scalariform. 



completely absorbed, leaving only a ring on the vessel wall to mark where 

 they had been. Examples of this are common among herbaceous 

 plants (Fig. 880). 



The lateral walls of vessels in the secondary wood may bear pits which 

 are either scalariform or circular, the latter being either side by side or 

 alternating with each other. Spiral vessels also occur in the wood of 

 many famihes, but not in those which are regarded as being the most 

 primitive. 



Movement of water upwards in the xylem under the influence of trans- 

 piration tension follows the lines of least resistance and large, fully open tubes 

 must obviously afford an easier passage than narrow elements with many 

 cross w^alls, even if the latter are perforated. 



