542 THE AMERICAN NATURALIST. [Vol. XXXVIII. 



originated schizogenously. About this the resin cells are gener- 

 ally flattened radially and disposed in such a manner as to sug- 

 gest the future development of a definite, limiting layer or 

 epithelium. In the completed form of the structure the central 

 space has broadened out and taken a circular form, assuming the 

 character of a definite cyst bounded by as definite a limiting 

 epithelium in which the cells are always flattened radially and 

 disposed concentrically (Fig. 43, C). Externally to these cells 

 there may be a second layer of similar resin cells, constituting 

 the outer epithelium, while the whole is enclosed on three sides 

 by a layer of parenchyma tracheids which are exceedingly like 



the associated tracheids of the spring wood, but from which they 

 may usually be distinguished by (i) their greater size and rela- 

 tively thinner walls, (2) the occurrence of bordered pits on the 

 tangential and terminal, as also upon the radial walls. Such 

 parenchyma tracheids never occur in the adjacent summer wood 

 for very obvious reasons, but on the radially opposite side of the 

 reservoir they are very commonly flattened radially (Fig. 43). 

 and they not infrequently present the same structural aspects as 

 the epithelial cells. The interchangeable relation between resin 

 cell and parenchyma tracheids as already shown would lead us to 

 suspect a substitution in the composition of the epithehum, and 



