392 ROYAL SOCIETY OF CANADA 
the thylose, for, on changing the focus, they stood out much more clearly. 
Proceeding towards the posterior end it is found that the thyloses 
become more and more frequent, canals entirely free from them being 
rarely observed, although canals that are only partly blocked occur con- 
tinually. The canal thus persists to a considerable extent, and is practi- 
cally as constant a feature as the thyloses, proving that the thyloses as 
a whole do not exhibit as active growth in this organ as in the stipe. 
The individual thylose cells also become thicker-walled as a rule, this 
no doubt being due to their more mature condition in this older portion 
of the rhizome. 
Another feature which is probably dependant on the greater age of 
the thyloses in this region is that starch becomes more and more abup- 

Fig. 24.—Pteris aquilina, rhizome. Fig. 25.—Pteris aquilina, rhizome. Thick 
Thick-walled thylose with long pits walled thylose, Pth., with pits on 
approaching scalariform markings. longitudinal and _ transverse y walls 
x 490. x 490. 
dant in them as we appoach the posterior end, the protoplasm becoming 
relatively more scarce. That starch should be present in the thyloses 
here is not at all surprising, for it is present in large quantities in the 
parenchymatous cells from which they originate. 
Thyloses with markings on their walls due to unequal thickening 
also become more abundant in this region, the markings being reticul- 
ated, scarlariform, or pitted. The latter are, however, by far the most 
prevalent. In some cases only the transverse walls exhibit these mark- 
ings (Fig. 24), whilst in other cases they are present on both the trans- 
verse and the longitudinal walls (Fig. 25). When there are pits on the 
transverse walls, the whole appearance greatly resembles a sieve-plate 
(Fig. 17). The thyloses thus become modified into vessels similar to 

