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. 35, E, F). 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, e.g. 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 

 Leptosporangiatse. Another group of ferns, intermedi- 

 ate in some respects between the lower (Eusporangiatse) 

 and the leptosporangiate ferns, are the Osmundaceee, 

 including the royal fern Osmunda regalis, and in east- 

 ern America the common cinnamon-fern, 0. cinna- 

 momea. In these the sporangia and the tissues, 

 especially the vascular bundles, show undoubted re- 

 semblances to the Eusporangiatse, 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, t) 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 





