124 



ANATOMY OF THE GYMNOSPERMS 



more or less extended and continuous compound reservoir, lying 

 tangentially. In their most rudimentary forms they present the 

 aspect of simple aggregates of resin cells without any differen- 

 tiation of a resin sac or of an epithelium. In a more advanced 

 stage of development there is produced a central cavity in the 

 form of an intercellular space (fig. 38, C.') which has obviously 

 originated schizogenously. About this the resin cells are gen- 

 erally flattened radially and disposed in such a manner as to 

 suggest the future development of a definite, limiting layer or 



epithelium. In the 



M ' R ' r\/fV completed form of the 



. E. :\\, structure the central 



space has broadened 

 out and taken a cir- 

 cular form, assuming 

 the character of a defi- 

 nite cyst bounded by 

 as definite a limiting 

 epithelium in which 

 the cells are always 

 flattened radially and 

 disposed concentric- 

 ally (fig. 38, C.). Ex- 

 ternally to these cells 

 there may be a second 

 layer of similar resin 



-4S.W 



FIG. 38. SEQUOIA SEMPERVIRENS. Transverse sec- 

 tion showing two contiguous resin cysts : C., com- 

 pleted and with a normal epithelium (E.) ; C/, an 

 intercellular space as the rudiment of a cyst with 

 imperfectly developed epithelium; M.R., the 

 medullary ray ; S. II'., the summer wood, x 225 



cells, constituting the outer epithelium, while the whole is inclosed 

 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 relatively thinner walls, and (2) the occurrence of bor- 

 dered pits on the tangential and terminal as well as upon the 

 radial walls. Such parenchyma tracheids never occur in the adja- 

 cent summer wood for very obvious reasons, but on the radially 

 opposite side of the reservoir they are very commonly flattened 

 radially (fig. 39), and they not infrequently present the same 



