[KIRSCH] CERTAIN STRUCTURES IN THE PTERIDOPHYTES 383 
the other hand, represent cells of later development, which either grew 
out from the sides of the canal, or which, in some cases were cut o!f 
from the vessel itself during its active growth, for the thyloses exhibit 
great powers of division. In some cases, however, it is just possible 
that some of the epithelial cells represent the original parenchyma cells 
which lined the canal, and which failed to produce thyloses. This, 
however, would only be possible in cases where the thylosal patches are 
fairly small, or where the vessel occupies a somewhat lateral position 
(Photo 8). 
The cysts most probably owe their origin to a process of arrested 
development. The thyloses in this instance would probably all com- 
mence to develop at about the same time, but for some reason or other, 
this growth into the canal would cease, and there would then ensue a 
structure as described above. 
In these cysts, however, there is a possibility of one of the cells 
subsequently growing out into the cavity, and occupying all the avail- 
able space, thus answering in all respects to a vessel. This fact may 
explain some of the cases where a vessel seemed to have a thinner wail 
than the surrounding epithelial cells. 
The probable function of these structures is more difficult of ex- 
planation. They, of course, bear a great resemblance to the resin and 
mucilage passages of certain Gymnosperms and Pteridophyta, but such 
substances were not shown in these sections by means of the ordinary 
tests. However, the material with which the tests were made, was not 
very favorable for demonstrating the presence of such substances, since 
it had been preserved in formaline for a considerable time, and also 
passed through water several times. Thus any mucilage or resin that 
may have been present in the fresh material would probably have been 
washed out, especially since they would most likely not have been pre- 
sent in more than minute quantities in the first place. This is there- 
fore a question which must be left for future investigation. 
Again these structures may be either manifestations of atavism, 
i.e. recurrence to a former state when canals or reservoirs of the same 
nature as the mucilage passages of the Marattiaceæ were present in 
these ferns, or they may be manifestations of mutation, i.e. a tendency 
to the formation of structures with secretory functions. 
Very little difference was noted in the character of the thyloses in 
proceeding from the tip to the base, except that in the latter region the 
thylosal patches as a whole were considerably smaller than in the parts 
above, the component cells also being more uniform in size and smaller 
(Photo 10). 
