54 



THE ANATOMY OF WOODY PLANTS 



elements from the tracheary and other fibrous constituents of the 

 secondary wood in the dicotyledonous angiosperms is sufficiently 



authentically supplied by the grouping of such 

 elements in longitudinal terminally pointed 

 groups possessing the exact configuration of 

 tracheids or fibers. Fig. 40 illustrates this 

 situation for the wood of the alder. To the 

 right and left are seen fiber-tracheids which 

 constitute the mechanical elements of the wood 

 in this genus. Between these two cells lies a 

 file of parenchymatous elements of the wood. 

 It is clear that the latter, represented with 

 heavy black boundaries, form a series which 

 both in length and in contour corresponds with 

 the adjacent fibrous elements. Normally there 

 are no transitions from tracheids to parenchy- 

 matous cells in the case of the dicotyledons, and 

 even experimental means do not in general 

 suffice to bring about the clear production of 

 parenchyma as the result of the progressive 

 septation of the original tracheary elements of 

 the wood. Some information on this subject, 

 however, is furnished by the phenomena of 

 injury in Liquidambar and Prunus. Here, as a 

 response to experimental or accidental injuries, 

 tangential rows of gum or mucilage canals are 

 formed in the wood. About these canals may 

 sometimes be detected short elements which 

 grade from abbreviated tracheids, resulting from 

 the septation of tracheids of normal length, to 

 typical parenchymatous elements, characterized 

 by persistent protoplasmic contents and simple 

 pits in their walls. 

 FIG. 40. Longi- Certain general characteristics of wood which 



tudinal view of depend as much as anything on the condition 



tracheids and paren- ^ d nch ma be conveniently 



chyma from the root J J J 



of the alder. introduced here. Fig. 41 is a photograph of the 



