Nos. 45I.-452.] A/VATOJfV OF THE CONIFER ALES. 



533 



are typically developed in Sequoia, Abies and Tsuga. They 

 always form a continuous series extending in a direction parallel 

 with the axis of growth ; but as the type of reorganization ad- 

 vances they merge, forming a continuous- canal such as may be 

 found typically in Pseudotsuga or Pinus. l-rom these state- 

 ments, then, it is clear that the parenchymatous resin cells 

 undergo modification in two directions, passing into i);neneliyma 

 tracheids, on the one hand, and on the other heeonun- shorter 



and shorter, accorcUng to conditions of aggregation, until they 

 pass into short cells which eventually constitute the epithelium 

 structure of the somewhat complicated resin passage, the latter 

 thereby becoming the expression of a peculiar aggregation of 

 resin cells. Whatever the stage of development may be, the 

 resin passage is always found to be composed of structural ele- 

 ments arranged in the following order from without toward the 

 center: (i) parenchvma tracheids, (2) resin cells eventually 



