364 ROYAL SOCIETY OF CANADA 
recent ferns, for in these all the wood parenchyma, and phloem cells are 
long and narrow excepting the thyloses which are found in the position 
of the protoxylem. 
In Plate XVI, Fig. 22, a similar bundle is shown, but it has no thy- 
loses in the large vessels. However, at the side of the vessels, there is 
a patch of cells identical with that marked g in Fig. 20 and which no 
doubt represents thyloses. This would afford an example of a fossil 
form showing the same structure as a normal recent form, since the 
thyloses in this case are only in the protoxylem region. Williamson 
says: “The presence of the thyloses, therefore, would seem to be an 
accidental phenomenon, and not a specific feature of the former speci- 
men,” meaning by this the thyloses in the vessels of the specimen shown 
in Fig. 20. That this statement is most likely correct is demonstrated 
by the phenomenon observer in recent ferns. 
Williamson also observed thyloses in a third form in the inner or 
non-exogenous zone of a yet undescribed species of Lepidodendron (28, 
318, Plate XVIII., Figs. 13 and 14). 
Weiss (25, 85), in a discussion on the thyloses of Rachiopteris cor- 
rugata, mentions the occurrence of thyloses both in the tracheids of the 
stem and also in one of the leaf-trace bundles of Zygopteris Grayin. In 
this paper he also describes what he terms “anomalous thyloses” (25, 
84, Figs. 12 and 13) which differ from the ordinary thyloses in having 
their walls thickened (probably lignified), and the thickening has taken 
place in such a manner as to give the structures the appearance of small 
pitted tracheids.” These “ anomalous thyloses” will be shown to be of 
frequent occurrence in recent ferns, being merely a modification of the 
normal thin-walled thyloses. 
References to canals of the same nature as those to be described 
below for Pteris aquilina are constantly found in text-books of fossil 
botany, e.g., Scott (17, 21, 88, etc.), Seward (18), etc., and tend to 
prove that physiological processes in the Carboniferous and other ages 
were governed by the same laws as at present. 
A reference of special significance is found in Seward (18, 321), 
In discussing the structure of Calamites, he says: “ An outgoing branch, 
as seen in a tangential section of a stem, consists of a parenchymatous 
pith surrounded by a ring of vascular bundles, in which the character- 
istic carinal canals have not yet. been formed, but if the section has cut 
the branch further from its base, there may be seen a circle of irregular 
gaps marking the position of the carinal canals. Such gaps are often 
occupied by thin parenchyma, and contain protoxylem elements.” In 
the light of what is to be described below it will be seen that different 
appearances at different levels represent consecutive stages of the same 
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