FIBROVASCULAR TISSUES: VESSELS 



101 







with the libriform fibers are devoid of both pits and spiral internal 

 sculpture. 



It will be profitable to consider next the type of vessel charac- 

 teristic of the older wood of the stem of Lirioden- 

 dron. This category of vessel is covered with pits 

 in horizontal rows, indicating contact with another 

 vascular element. Below and above the sharply 

 sloped end walls are seen; these are neither 

 porous, as in the case of the vessels of the last 

 figure, nor marked by the simultaneous presence 

 of narrow scalariform perforations and bordered 

 pits, as in the rootwood of Magnolia and in the 

 general secondary wood of many of the Betu- 

 laceae, etc. The terminal apertures of the vessel, 

 in fact, are crossed by a few remote bars. The 

 type of vascular element characteristically present 

 in the old wood of Liriodendron serves, indeed, as 

 an intermediate stage between the vessel with true 

 scalariform perforations and" that with porous 

 terminal apertures. 



The truth of this assertion becomes manifest 

 from a consideration of Fig. 79, which represents 

 vascular structures from the wood of the high blue- 

 berry, Vaccinium corymbosum. In a, b, and c are 

 shown successive transitions from end walls with 

 many pits and few narrow scalariform perforations 

 to fewer pits and more scalariform apertures, and 

 to a third condition where the pits have dis- 

 appeared and only somewhat wide scalariform 

 openings remain. In d and e appear further 

 stages in which the bars separating the terminal 

 large slits become fewer and more degenerate until 

 finally a simple porous condition is reached. It is 

 thus clear that the vessel with porous perforations 

 is a further elaboration of that with scalariform 

 perforations, just as this in turn has taken its origin from a vascular 

 element with pitted perforations of the nature found in the Gnetales. 



FIG. 78. Ves- 

 sel from the stem 

 of Liriodendron. 



