312 MOSSES AND FERNS chap. 



basipetal succession on the receptacle. The remaining sub- 

 families of the Polypodiacese constitute the suborder, "Mixta;," 

 in which sporangia of very different ages are mixed together in 

 the same sorus. 



The well-known Ostrich-Fern, Onoclea struthiopteris 

 (Struthiopteris Germanica) illustrates very satisfactorily the 

 germination of the spores and the development of the gameto- 

 phyte and embryo in the Polypodiacese, the typical modern 

 Ferns. O. sensihilis, which may probably be better separated 

 generically from Struthiopteris, agrees closely with the latter in 

 the development of the gametophyte. 



The large oval spores contain, besides much oil and some 

 starch, numerous small crowded chloroplasts. The three walls 

 of the spore are plainly demonstrable, especially as the brown 

 perinium is often thrown off by the swelling of the spore, and 

 the transparent exospore can then be seen, with the delicate 

 endospore lying close to its inner face. A large nucleus 

 occupies the centre of the spore. Contrary to the statements 

 usually made that spores containing chlorophyll quickly lose 

 their vita,lity, these will germinate after a year or more, although 

 not so well as those of the same season, but they normally 

 remain from autumn until spring before they germinate. O. 

 sensihilis acts in the same way, and spores of other Ferns con- 

 taining chlorophyll have been germinated after an equally long 

 period. 



The spores germinate promptly, varying from two or three 

 days to about a week, depending upon the temperature. The 

 exospore is ruptured irregularly near one end, and through this 

 a short colourless papilla protrudes and is shut off by a trans- 

 verse wall (Fig. 173, B). This papilla contains little or no 

 chlorophyll and rapidly lengthens to form the first rhizoid, 

 which undergoes no further divisions. The large green cell: 

 alone produces the prothallium. The divisions in the pro-' 

 thallial cell vary somewhat, but in the great majority of cases a 

 series of transverse walls is first formed, and the young pro-'- 

 thallium (Fig. 173, C) has the form of a short filament. 

 Sooner or later, in normally-developed prothallia, the terminal' 

 cell of the row becomes divided by a longitudinal wall, which ' 

 may be straight, but more frequently is oblique and followed 

 by another similar wall in the larger of the two cells, meeting it 

 so as to include a triangular cell, which is the "two-sided" apic?tl^ 



