126 



ANATOMY OF THE GYMNOSPERMS 



are placed in vertical series of indeterminate extent, but at varying 

 intervals of such a nature that they may sometimes be separated 

 only by a rather thick wall of short resin cells. At other times 

 they are somewhat distant and separated by an extensive vertical 

 tract of resin cells. From this it is obvious that in any given 



plane of section there 

 will be a great diversity 

 of aspects presented, but 

 in the main exhibiting 

 structural gradations in 

 the development of the 

 reservoir, as already re- 

 counted. In some cases 

 thick-walled cells of cir- 

 cular outline may be seen 

 S^^TS-^ in transverse section to 



CD 







c.. 



S.W.T 





 



stand out from the gen- 

 eral line of the epithelium 

 and lie within the cavity 

 proper. More rarely such 

 cells are so multiplied as 

 to fill the entire cavity, 

 and they may themselves 

 be filled with granular 

 resin. Such features are 

 clearly defined (fig. 39), 

 and it is evident from the 

 way in which such cells 

 originate from the epithe- 

 lial cells that they are of 

 the nature of thyloses. A longitudinal section through such a 

 reservoir (fig. 41) shows how such thyloses occupy the entire 

 cavity of the cyst, while in other cases they may be purely local 

 (fig. 40). Among fossil Sequoias similar thyloses form a most 

 characteristic feature in the resin passages of the medullary rays 

 in S. Burgessii and S. Penhallowii. 



FIG. 40. SEQUOIA SEMPERVIRENS. Radial sec- 

 tion of a resin cyst showing the epithelium 

 (ep.) ; the central cyst (c.) with a thylosis (///.) ; 

 parenchyma tracheids (pr.t.),a.nd a tracheid 

 of the spring wood (Sp. 7 1 .). X 300 



