[KIRSCH] CERTAIN STRUCTURES IN THE PTERIDOPHYTES 379 
is not a frequent occurrence in the mature stipes. This cavity is merely 
a space between the thylose cells, and wherever observed is usually filled 
up with remains of the disorganised protoxylem elements. This appear- 
ance tends to prove that wherever these loose rings and spirals are visible 
in the thylose groups, they are included in the intercellular spaces, and 
not in the cells themselves. 
Besides presenting differences in the thickness of their walls, in 
size, and in freedom from contents, the thyloses in these sections exhibit 
certain modifications which manifest tendencies towards the formation 
of definite vessels or cysts with epithelia. Im many cases an appear- 
ance which attracts one’s attention in a group of thyloses, is a large, 
round, more or less thick-walled cell, clear of contents, and occupying 

Prx----- 
Fig. 8.—Pteris aquilina. Stipe, Oct. 25th, 1906. Fig. 9—Pteris aquilina. Stipe, Oct. 25th, 
Individual thylose, Th., forming a thic*- 1905. Thyloses forming a vessel with an 
walled vessel. x 367.5. irregular epithelium. x 367.5. 
a central position in respect to the other cells of the group. This cell 
is usually as large or larger than the largest vessels of the secondary 
xylem, and is surrounded by other thylosal cells, which, although they 
differ greatly in size and in regularity of shape, still present the appear- 
ance of an epithelium. In our description of these modifications we 
shall therefore designate the structure as a vessel with epithelium. 
The size of the vessel and the regularity of the epithelium vary 
greatly, as is partially shown by the accompanying drawings and photo- 
graphs. In figures 8 and 9, although the vessels are clearly defined, the 
surrounding cells can only be termed an epithelium in the widest sense 
of the word. 
