[KIRSCH] CERTAIN STRUCTURES IN THE PTERIDOPHYTES 377 
thylose cells surrounding it (Fig. 6). In some cases the central cell was 
observed to be almost perfectly circular, and absolutely clear of contents, 
and thus presented the appearance of a central vessel or sac with an 
epithelium. This appearance is very frequent in still older stipes, and 
will be dealt with more fully below. 
In these stipes very little change in the character of the structures 
was noticed in proceeding from the tip to the base, the patches present- 
ing a similar appearance. The individual thylose cells showed more or 
less disorganised, granular, cytoplasm with nuclei. 
The oldest stipes examined, which were collected October 25th, 
1906, exhibited many interesting features. Before proceeding with an 
account of these, however, it will be well to notice the condition of the 
plants at the time they were picked. Winter was just setting in, and in 
many cases the stipes were already completely gone. This was especially 
the case with the smaller specimens. Others again, when cut in the 

Fig. 6.—Pteris aquilina. Stipe, Sept. 23rd, 1906. Showing Thylose, Th., modi- 
fied to form sac or vessel. X 490. 
field, showed a great degeneration of the tissues, as manifested in the 
easy rupturing of the component cells .and in the icy contents, the ice 
probably resulting from the freezing of the water in the vessels and 
other cells. 
Some of the largest stipes, however, were in a fairly good state of 
preservation, and aside from manifesting a decided tendency on the part 
of the epidermal tissues to break in cutting, afforded very good sections 
of the vascular bundles. Thus the stipes to be discussed were all of 
vigorous growth, and measured about 40 cm. in length, with a mean 
diameter of about 0.7 cm. 
One of the most noticeable features in sections of these stipes, is 
the great tendency to fusion exhibited by the bundles, so that it is usual 
to find very long, narrow, strap-shaped bundles mingled with the smaller 
