[KIRSCH | CERTAIN STRUCTURES IN THE PTERIDOPHYTES 381 
In all the cases described above, there is a definite central vessel sur- 
rounded by a more or less regular epithelium. In other cases, however, 
which presented an analogous appearance, it was found that there was 

Fig. 11.—Pteris aquilina, stipe, Oct. 25th, 1906. Portion of canal, C., modified 
to form a cyst. Some of the epithelial cells, Ep., exhibiting thylosal 
growth. X 367.5. 

Fig. 12.—Pteris aquilina, stipe, Oct. 25th. 1906. Showing cyst, C., with 
regular epithelium, Ep. X 367.5. 
no central cell or vessel, the epithelium bounding a cavity that possessed 
no wall of its own (Figs. 11 and 12). Some of the bounding cells in. 
