134 EVOLUTION OF PLANTS 
Ophioglossum. This specialization consists in a more 
definite limitation of the sporangium, and the restric- 
tion of the primary sporogenous tissue or archesporium 
to a single cell (Fig. 85, E, FP). The genus Botry- 
chium, which is obviously related to Ophioglossum, 
has, within its limits, species which illustrate the 
change from a large indefinite sessile sporangium much 
like that of Ophioglossum, eg. B. simplex, to the 
much smaller obviously stalked sporangium found in 
such large species as B. Virginianum (Fig. 34, D). 
This evolution of the sporangium is accompanied by a 
growing complexity in the divisions of the leaf, as well 
as correlated modifications of the tissues of the sporo- 
phyte, which approximate the structures of the typical 
Leptosporangiate. Another group of ferns, intermedi- 
ate in some respects between the lower (Eusporangiate) 
and the leptosporangiate ferns, are the Osmundacee, 
including the royal fern Osmunda regalis, and in east- 
ern America the common cinnamon-fern, O. cinna- 
momea. In these the sporangia and the tissues, 
especially the vascular bundles, show undoubted re- 
semblances to the Eusporangiate, although, on the 
whole, they are nearer the leptosporangiate type. 
In the latter the sporangia can be traced back to a 
single epidermal cell, the early divisions of which are 
extremely regular, and result in the formation of a 
single central archesporial cell surrounded by a single 
layer of outer cells. The so-called “tapetal cells” 
(Fig. 35, F, ¢) are cut off from the archesporium, but 
are later broken down, so that at maturity the wall of 
the sporangium consists of a single layer of cells. A 
constant character of these ferns is the formation of 
