[KIRSCH] CERTAIN STRUCTURES IN THE PTERIDOPHYTES 371 
swelling continues they grow into the cavity, the individual cells grow- 
ing as a whole and not more actively at one point than at another. Thus 
there is no formation of papillæ or tubular outgrowths from these cells 
as Thome records (22, 125), tne wail of the free end of the cell jutting 
out into the cavity as a whole, and thus merely extending and enlarging 
the original cell. In this manner the cells step out of the limits of their 
tissue and grow out towards the centre of the canal in all directions, 
In many cases one cell outstrips the others a great deal, so that frequently 
a single, large, bladder-like cell is seen partly filling the canal (Fig. 2). 
In all these cases the ceil is perfectly rounded, and shows no pseudo- 
podial projections from the sides, as would be expected if it had the 

Fig. 2.—Pteris aquilina, Stipe, base. Collected May 29th, 1899. Showing 
thylose, Th., growing out into the canal, C. x 490. 
power to put out papillar or tubular prolongations as described by 
Thome; for there is a free space around the whole cell, except at the 
point of attachment to its original tissue, where it could put forth these 
protrusions. 
The phenomenon described above is none other than the formation 
of thyloses,— structures characteristic of Mono-, and Dicotyledons, and 
recorded only in some fossil ferns. 
Weiss, in his paper “On the Thyloses of Rachiopteris Corrugata ” 
(25, 82, April 1906), says: “But in the two fern-remains mentioned 
the general appearance of the blocked-up tracheids is so remarkably like 
that of vessels filled with thyloses, that one is inclined to accept William- 
son’s conclusions in spite of the fact that such thyloses have not been 
