336 



THE ANATOMY OF WOODY PLANTS 



three remaining genera of the Pineae the lining of the secretory 

 space is composed mostly of thick-walled more or less lignified 

 cells. The resin canals in these forms do not accordingly contain 

 well-developed tyloses in the heartwood. Another important 

 distinction between Pinus and allied genera is the complete absence 

 of wood parenchyma in the former. It has been shown in an 

 earlier chapter that longitudinal storage parenchyma is formed 

 in the secondary wood as the result of the modification of elements 



FIG. 244. Wood of Picea canadensis, showing terminal parenchyma 



destined to be tracheids. In Picea, Larix, and Pseudotsuga storage 

 parenchyma is present, but at the end of the annual ring only 

 (Fig. 244). In this position, particularly in the case of the root, it 

 manifests convincing evidence as to its derivation by the occurrence 

 of transitional stages between merely septate tracheids and rows of 

 parenchymatous elements, resembling in their general configuration 

 tracheary elements. In Pinus, therefore, there is no true paren- 

 chyma of the wood, since such storage cells are found only in the 

 three other genera of the Pineae. Where wood parenchyma is 

 present, moreover, it is confined to the end of the annual ring and 

 is clearly in a condition of derivation from tracheids, a state found 

 normally in no other living representative of the Coniferales. It 



