392 MOSSES AND FERNS CHAP. 



The position of the sori is that of the typical Polypodi- 

 aceae, and sometimes a decidedly elevated placenta is present. 

 The indusium is either cup-shaped (Cyathea), or bivalve, e. g., 

 Cibotium (Fig. 229). In the latter the outer valve fits closely 

 over the other like the cover of a box. The sporangia which 

 are either long or short-stalked, although their development 

 has not been followed, correspond so closely in the mature state 

 to those of the Polypodiaceae that there is little doubt that their 

 development is much the same. The annulus is nearly or quite 

 complete, but above the stomium in Cibotium Mensiesii the cells 

 of the annulus are broader but thinner-walled (Fig. 2-27, C), 

 and Atkinson shows much the same appearance in C. Chamissoi. 

 In the former species the stalk is long and composed of three 

 rows of cells, as in typical Polypodiaceae. With the sporangia 

 in this species are also numerous long paraphyses (Fig. 

 227, D). 



THE PARKERIACE^: (Diels (i), Kny (6)) 



This family comprises but a single species, Ceratopteris 

 thalictroides, a peculiar aquatic Fern of wide distribution in 

 the tropics. Unlike most'Pteridophytes, Ceratopteris is char- 

 acteristically annual, although by the formation of adventive 

 buds it may become perennial. 



The prothallia are usually dioecious, and the antheridia dif- 

 fer from those of the typical Polypodiaceae in projecting but 

 little above the surface of the prothallium. 



Except for the peculiarities due to its aquatic habit, in which 

 respect it differs from all other homosporous Ferns, the growth 

 of the organs and structure of the tissues is similar to those of 

 the Polypodiaceae, to which family Ceratopteris is often as- 

 signed. 



The development of the sporangium is essentially like that 

 of the Polypodiaceae, but the annulus sometimes shows an in- 

 complete development, probably correlated with the aquatic 

 habit of the plant (Hooker (i), p. 174). 



THE POLYPODIACE^: 



The Polypodiaceae may very aptly be compared to the stego- 

 carpous Bryineae among the Mosses, inasmuch as like that 



