RESIN PASSAGES 129 



usually recognizable by their rather unusual size and thinner 

 walls, and in addition they commonly show bordered pits on the 

 tangential walls. When the terminal wall lies sufficiently near 

 to the plane of section it shows from one to several rather large 

 bordered pits, and by this feature such tracheids may be located 

 with much certainty. Thyloses have been definitely noted only 

 in the case of A. concolor, in which species they are essentially of 

 the same general character as in Sequoia. They are thick-walled 

 and either isolated or so numerous as to fill the entire cyst. In 

 one case of contiguous cysts an epithelial cell was found to form 

 thyloses in both cysts, - - in the one case giving rise to an isolated 

 cell, in the other forming a tissue which nearly filled the entire 

 cavity. 



In radial section the reservoirs are round or oblong cysts of 

 variable size, and they are either contiguous or distant. In the 

 former case they rarely or never become confluent, but they 

 maintain their separate identity, as in Sequoia and Tsuga. In 

 the latter case the intervening region is occupied by an aggre- 

 gate of resin cells in all essential respects like those in the same 

 regions of Sequoia and Tsuga Mertensiana, or like the aggre- 

 gates which are generally characteristic of Tsuga caroliniana. 

 The inner epithelium usually consists of short cylindrical and 

 strongly pitted cells, which in the second and third rows become 

 successively longer and less strongly pitted, so that those in the 

 outer row may be identical in form and markings with the isolated 

 resin cells. In the two outer rows the cells not infrequently 

 show bordered pits on their radial walls, thus presenting transi- 

 tional forms which gradually pass over into tracheids, and the 

 one then replaces the other. The parenchyma tracheids, which 

 are always most characteristic of the spring wood, are always 

 distinguished by the presence of large and prominent bordered 

 pits, but in addition they are sometimes broad and thin-walled 

 and lie in radial series. 



From these facts it is clear that the secretory reservoirs of 

 the three genera in question always take the form of closed 

 sacs, which De Bary has already pointed out as a feature of 



