Osmunda claytoniana , Z., ( 9 . cinnamomea , Z. 75 
sections cut parallel with the surface of the prothalliurn. As 
the cotyledon lengthens, it flattens out somewhat, and the three- 
sided apical cell is probably replaced by a two-sided one, as 
in the Polypodiaceae, but this point was not positively proven. 
As it becomes older, the primary tissue-systems become 
differentiated, but are not as prominent as is usually the case. 
The plerome-cylinder seems to arise exclusively from the 
innermost of the two cells into which each segment of the 
apical cell is at first divided ; while the outer and larger one 
gives rise to dermatogen and periblem. The former consists 
of a single layer of pretty well-marked, nearly iso-diametric 
cells, which undergo no further periclinal division. The peri- 
blem consists of about three layers of cells that increase a 
good deal in length as the cotyledon lengthens. The plerome, 
as usual, forms a central cylinder of cells, in which the longi- 
tudinal divisions predominate, so that they are much elon- 
gated. The nuclei of these cells are also elongated, instead of 
round, as in the dermatogen and periblem. 
As the leaf continues to grow, it curves strongly backward, 
and on account of its position grows out laterally, lying close 
against the under side of the prothallium. Owing to its 
varying position, however, it is difficult to get perfectly straight 
sections, and in consequence it is not easy to determine posi- 
tively the exact form and divisions of the apical cell. There 
is probably here, as in Polypodiaceae, a true dichotomy of the 
apical cell, giving rise to the dichotomous branching of the 
veins, but owing to lack of material this point was not investi- 
gated. But as the appearance and venation of the full-grown 
cotyledon corresponds closely with that of the Polypodiaceae, 
there is no reason for doubting that this is the case. 
As the cotyledon begins to lengthen, short glandular hairs 
develop near the apex .(Fig. 100), and probably serve for 
protection, as is so often the case in the older sporophyte 
of most ferns, including Osmunda. These hairs are short, 
usually but two cells long, the upper somewhat enlarged and 
secreting probably a mucilaginous matter. These cells stain 
very strongly with Bismarck-brown. 
