{KrrscH] CERTAIN STRUCTURES IN THE PTERIDOPHYTES 389 
of a horse-shoe, the protoxylem elements lying on the inside of the con- 
cavity. The protoxylem, however, takes up only a small portion of the 
area enclosed by the horse-shoe, the remainder being occupied by wood- 
parenchyma and phloem. Hence, when the canal begins to form, it has 
a considerable area available for lysigenetic degeneration through the 
rupturing of the wood parenchyma cells, and in this manner its bound- 
aries are extended. (Compare Photos 6 and 12.) 
Commencing at the anterior end, it was found that thyloses were 
present in many of the bundles, but that they differed considerably from 
those observed in the stipe. They were about the same size as the neigh- 
boring parenchyma cells and very rich in contents, so that it was rather 
difficult to distinguish them except by their position and by their more 
or less irregular shape (Fig. 18). It was thus found advisable to clear 

Fig. 18.—Pteris aquilina. Rhizome, tip. Thyloses, Th., adjacent to proto- 
xylem, Prx.. X 490. 
the sections with 1% potassium hydrate to remove the cell contents, a 
parallel series of sections with contents also being examined. When 
treated in this manner, the structures stood out very distinctly, and it 
was observed that the canals in bundles of the same transverse section 
did not exhibit parallel phases of development. 
Various gradations in the structure were represented; from places 
where only the open, irregular, canal was present, to places where the 
canal was blocked by thick-walled thyloses. The canals also varied 
greatly in size and appearance, from mere irregular chinks to compar- 
atively large cavities with a definite lining of parenchyma and proto- 
xylem cells. Some of these stages are depicted in the following figures. 
