384 ROYAL SOCIETY OF CANADA 
In longitudinal sections taken at the base, thyloses which have 
fairly thick walls were seen to fill the canal almost completely, leaving 
only a few small lacunæ here and there (Fig. 14). In figure 15 a sec- 
tion from the same region exhibits a curious modification in some of 
the thyloses. In these there are localised thickenings in the wall which 
give the cells the appearance of pitted tracheids. The pits are spindle- 
shaped and are uniformly distributed on the surface of the wall. Modi- 
fied thyloses of this character are rarely found in the stipe, but, as will 
be seen below, are of common occurrence in the rhizome. 










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Fig. 14.—Pteris aquilina, stipe, Oct. 19th, 1906. Longit. Canal blocked with 
thyloses, Th. X 367.5. 
.Thyloses of this nature are met with in fossil ferns, e.g. in Kachio- 
pteris corrugata, and Weiss (25, 85) speaks of them as follows:— 
Serko strate , in this particular instance the cellular protrusion differs from 
the ordinary thyloses in having its walls thickened (probably lignified), 
and the thickening has taken place in such a manner as to give the 
structure the appearance of a small pitted tracheid. As the pits are of 
irregular shape and somewhat pointed, the marking at first sight looks 
slightly spiral. This is exactly the appearance of those described by 
Miss Jordan, and there is thus a very close agreement ketween this 
thickened thylose of Rachiopteris corrugata and those from a section of 
Cucumis figured in Plate X. of Vol. II. of the New Phytologist.” That 

