380 ROYAL SOCIETY OF CANADA 
In Photo 7 there is a definite vessel, entirely surrounded by thylosal 
cells, but these cells are of various sizes, some of them being almost as 
large as the vessel itself, whilst others are many times smaller. How- 
ever, they can be properly termed an epithelium. In this photo it is 
noticeable that the epithelial cells take up the stain (hematoxylin), 
whilst the central vessel shows no sign of the stain whatsoever. This 
appearance is general in all the structures of this nature that came under 
observation. Figure 10 shows a very well-formed vessel with a typical 
epithelium, the component cells of the latter being of practically equal 
size and elongated in the direction parallel to the circumference of the 
vessel. 

Fig. 10.—Pteris aquilina. Stipe, Oct. 25th, 1906. Thylose functioning as a 
vessel and surrounded by a regular epithelium, Ep. X 490. 
In Photo 8 a large vessel surrounded by a most regular epithelium 
is shown. Part of the epithelium here lies between two large thylosal 
cells, and this gives the impression that the epithelial cells had bridged 
the original canal in order to completely surround the vessel. 
Photo 9 shows the same vessel in a section somewhat lower down 
the stipe. In this it is seen that although the vessel itself continued 
down to this point, the epithelium has lost its regular character, the 
whole structure assuming the appearance of an ordinary group of thy- 
loses to a considerable extent. Various intermediate stages in the re- 
gularity of this epithelium were observable in the few sections where the 
vessel was visible. . 
The vessels are much longer than the ordinary thyloses, for they 
can generally be identified in several consecutive sections. However, 
they are not as long as the xylem vessels, and partake more of the nature 
of sacs, such as the resin sacs which occur in certain Gymnosperms. 
TT 
