388 ROYAL SOCIETY OF CANADA 
tudinal section. Such pitted tracheids are, however, not very common 
“in this region. } 
The rhizome of Pteris aquilina extends through the soil for a con- 
siderable distance, and is often several metres long, branching frequently. 
The various branches wlich arise from it divide it into a large number 
of parts which are of unequal length. Since the parts, however, repre- 
sent equal periods of growth, they are consequently manifestations of 
regions of unequal activity during the formative period. 
In transverse sections of these rhizomes it'is evident that there are 
two systems of bundles—a central system and a cortical one. The bound- 
ary between these regions is determined by two long sclerenchymatous 
bands which lie above and below the central system of bundles—the 
bundles of the stem proper. The cortical bundles lie at the periphery, 
on the outside of the sclerenchymatous bands, and are the leaf-traces. 
There is a place on each side where the cortical parenchyma runs into 
the central parenchyma, between the bands of sclerenchyma, and these 
represent the foliar gaps through which the foliar bundles are given off 
from the bundles of the stem. 
The bundles of the two regions differ somewhat in appearance, the 
central ones being very long and with the xylem occupying by far the 
greater part of the bundle, whilst the cortical bundles, are as a rule, 
smaller and rounder with the xylem less extensive. The canals and 
thyloses that will be described below are never found in the central 
bundles, occurring only in the cortical leaf-traces. In the following 
description the oldest part of the rhizome, 1.e., where growth commenced, 
will be indicated as the posterior part, the region where growth termin- 
ated, as the anterior part. 
The cortical bundles, as a rule, showed either canals or thyloses, 
these occurring at one or two points in the bundle. The number varied 
as the shape of the bundle, and primarily as the number of the proto- 
xylem groups, for in these, as in the other regions, the canals and thy- 
loses are only found in the regions of the protoxylem. In bundles which 
are elliptical in outline, there are usually two canals or groups of thy- 
loses at the foci of the ellipse (Photo 12) ; in bundles of circular outline 
there is one central canal or group of thylosal cells. 
The canals and thylosal groups are of comparatively small size, and 
this is undoubtedly due to the character of the bundles in this region. 
The protoxylem elements in these bundles are surrounded on all sides 
by the secondary xylem, and have very little wood parenchyma mingled 
with them. Thus there is very little room available for the formation 
of the canal. In the stipe, on the other hand, the xylem has the shape 


