386 ROYAL SOCIETY OF CANADA 
which are above ground, and the underground parts will now be dealt 
with. 
The first part to be described here will be the subterranean portion 
of the stipe, which presents several significant features that are of value 
in explaining the whole structure. 
This portion of the stipe of Pteris aquilina differs both externally 
and internally from the ærial portion. Externally it is characterised 
by the black color of the epidermis, as contrasted with the light brown 
or reddish color of the mature ærial portion. Internally it is differen- 
tiated by the occurrence of patches of thick-walled, brown-colored, scle- 
renchymatous tissue, which lie between the bundles. At the base of this 
portion these sclerenchymatous groups are aggregated into a more or less 
regular T-shaped band, but as the top is approached the T becomes 
broken up into small portions which disappear altogether in the base of 
the ærial portion of the stipe. 
Besides this sclerenchyma there is a sheath of sclerenchymatous 
cells around each bundle, and the hypodermal layers of the whole trans- 
verse section consist of 6 or 7 layers of cells of the same character; the 
sections obtained from this region therefore present a dark brown ap- 
pearance. All this ligneous modification tends to make this part highly 
resistive to decay, and so stumps 7 or 8 years old are frequently observed 
that still have their tissues intact. 
Thyloses are not present in all the bundles of this region, and 
where they do occur they occupy only a small area. The individual thy- 
loses are generally very thick-walled, and of small lumen. The bundles 
are here also noticeable by the relatively large masses of wood-paren- 
chyma and phloem which occur on the inside of the xylem loop, these 
elements exceeding the area of the thylose groups many times. This 
appearance is not so difficult to understand, however, if it is borne in 
mind that this region really represents the bud of the stipe. 
Taking this fact into consideration it will be remembered that there 
was comparatively slow growth here, and that the canals were formed 
mostly in a schizogenous manner. Thus there was no extensive ruptur- 
ing of the wood parenchyma elements, such as occurs in the ærial portion 
of the stipe, and for this reason the canal and the resultant thyloses 
occupy a much smaller area. Hence this indirectly proves the important 
part played by lysigenous degeneration in the formation of the large 
canals of the ærial organs. In cases where the growth was very slow the 
canal did not form, at all, and so instead of a group of thyloses on the 
inner side of the xylem loop, there is found an aggregation of wood 
parenchyma and phloem. 

