[KIRSCH] CERTAIN STRUCTURES IN THE PTERIDOPHYTES 365 
structure, and that these stages are exactly paralleled in a growing 
stipe of Pteris aquilina. The thin parenchyma is none other than the 
thylosal tissue which fills up the canal shortly after its formation. 
An attempt having been made to summarise the various re- 
corded observations of the structures under discussion, both in recent 
and fossil Pteridophyta, an account will now be given of the results 
obtained in an examination of certain existing ferns. Before proceed- 
ing, however, it will be well to state here that all the work to be recorded 
in the following pages was done before any of the above articles were 
seen, and so no steps were taken to go over any of the work of the 
authors mentioned, and all the conclusions stated below were arrived at 
independently. 
In the course of the laboratory work on ferns in the McGill Uni- 
versity Botanical Laboratory, patches of large, irregular cells in the 
vascular bundles of the stipe of Pteris aquilina had frequently been 
noticed by Professor D. P. Penhallow; it was therefore decided to in- 
vestigate this appearance at the earliest opportunity. With this in view, 
a considerable amount of material, consisting of stipes of Pteris aquilina 
at different stages of growth, had been collected by him and formed the 
starting point of the following work. 
The material of Pteris aquilina collected by Professor Penhallow 
on May 29th, 1899, represented some of the earliest stages of growth in 
the stipe, and was supplemented by collections of the mature stipes, the 
buds, and the rhizomes obtained at different times in the autumn of 
1906. The specimens of Osmunda regalis, Onoclea sensibilis, and 
Asplenium spinulosum dealt with below were also collected in the 
autumn of 1906, all the other material examined being obtained from 
the collection of preserved plants in the Botanical Laboratory of McGill 
University. 
All the specimens were preserved in 5% formaline and sectioned 
by means of the freezing microtome. Delafield’s Hematoxylin, and 
Bismarck Brown were the only stains employed and afforded very satis- 
factory results. The drawings accompanying this paper were made by 
means of a Bausch and Lomb camera lucida, and the microphotographs 
were all taken by Professor Penhallow. 
