4 6 



THE ANATOMY OF WOODY PLANTS 



other than in the fact that they are found in spindle-shaped or fusi- 

 form groups resembling in contour tracheary elements. Injured 

 specimens of the woods of the four genera under discussion show 

 (Fig. 35), however, the transition from short tracheids to true 

 parenchyma cells as the result of the septation of elements laid 

 down by the cambium as tracheids. This is an example of the 



FIG. 35. -Injured wood of Tsuga canadensis, showing transition of tracheids 

 to parenchyma. 



recall of ancestral conditions as a consequence of injury, a phe- 

 nomenon extremely common in the case of plants with secondary 

 growth. This situation is classified, as will be shown later, under 

 the caption of the evolutionary doctrine of reversion. 



Already in the higher representatives of the pinelike conifers 

 namely, Abies and Tsuga the parenchyma tous elements have 

 not only lost all normal indications of derivation from tracheary 

 elements at the end of the zone of annual growth, but have even 

 begun to abandon the strictly terminal position in the yearly rings 



