FIBROVASCULAR TISSUES: PARENCHYMA 59 



they often clearly reveal their derivation from tracheids by almost 

 imperceptible transitions into elements belonging to this category. 

 The terminal situation of the primitive parenchymatous cells is 

 probably of advantage to the cambium awakening from its winter 

 sleep and standing much in need of instantly available food. In 

 general, the tangential terminal parenchyma of the more primitive 

 conifers has the same nutritive relation to the cambial elements as 

 the similarly located tangential pitting of the tracheids has to the 

 water supply of the cambial zone. 



Later the longitudinal elements devoted to the function of 

 storage, like the tangential pitting of the tracheids, became dis- 

 tributed throughout the annual ring. So long as there was no 

 further differentiation of the elongated elements of the wood there 

 was no incentive to further evolution on the part of the elements 

 of the wood parenchyma. With the appearance of the vessel as 

 the final expression of efficiency in the transport of water on the 

 part of the wood and the correlated gradual loss of the aquiferous 

 function on the part of the tracheids (which progressively gave rise 

 to fiber-tracheids and libriform fibers, respectively more and more 

 specialized in the mechanical direction), a new tendency found 

 expression in the organization of the longitudinally oriented 

 storage devices of the wood. In the case of the higher gymnosperms 

 (Gnetales) and lower dicotyledons the fibrous elements of the wood 

 are still largely capable, by reason of the presence of numerous 

 bordered pits in their walls, of the transport of water. The higher 

 dicotyledons, however, are in general characterized by the strict 

 allocation of the function of movement of water to the vessels, and 

 the tracheary structures of lower types become transformed into 

 purely mechanical or partially mechanical and partially food-storing 

 elements designated progressively as fiber-tracheids, libriform 

 fibers, septate fibers, and substitute fibers. With the appearance 

 of this situation the parenchymatous elements of the wood 

 become more and more relegated to the vicinity of the vessels for 

 their necessary supplies of all-important water. This situation 

 receives its final morphological and evolutionary expression in the 

 appearance of strictly localized vasicentric parenchyma in the case 

 of high dicotyledonous groups, such as the Compositae, Sapindales, 



