FILICINEAE.HOMOSPOROUS FILICINEAE. 



199 



a 



which is found in prothallia that bear archegonia; the} 7 are, as Prantl 1 has shown, 

 arrested forms, which in consequence of insufficient nourishment, and especially from 

 want of a due supply of nitrogen, are not fully developed, but produce instead a 

 large number of antheridia. These * ameristic ' prothallia may be transformed into 

 normal prothallia with archegonia, by supplying them with the necessary food. 



This typical course of development is 

 liable to certain variations, the more impor- 

 tant of which must now be mentioned. Even 

 in the Polypodiaceae, in Polypodium vulgare 

 for example, the formation of a cell-filament 

 is sometimes entirely dispensed with, and a 

 cell-surface proceeds immediately from the 

 germinating spore. This is the rule in 

 Osmunda, where a cell-surface is the first 

 product of germination, and, as in the 

 Equisetaceae, a posterior cell produces the 

 first rhizoid; sometimes, however, a short 

 cell-filament is first formed, as in the Poly- 

 podiaceae, or even a many-layered cellular 

 body. The rhizoids proceed from marginal 

 cells, and on the under side from surface 

 cells of the prothallium ; the apical growth 

 of the prothallium is similar to that of the 

 Polypodiaceae. A mid-rib formed of several 

 layers of cells is characteristic of the pro- 

 thallium of Osmunda (Fig. 148); this is con- 

 tinued from the posterior extremity to the 

 apex, and produces on its under side numer- 

 ous archegonia in two longitudinal rows ; the 

 antheridia, on the other hand, are not on 

 the mid-rib, but on the margin or on the 

 under surface of the prothallium. When 

 an oosphere has been fertilised in an arche- 

 gonium, the heart-shaped prothallium ceases 



to grow, and it disappears as the young plant developes. But if no oosphere is 

 fertilised, the prothallium continues to grow and becomes a ribbon-like body 

 with the aspect of Pellia one of the thalloid Hepaticae, and may live during 

 several years attaining a length of more than four centimetres. In this case certain 

 cells in the margin of the terminal sinus grow more vigorously, and lobes are 

 formed in. the prothallium, right and left alternately (Fig. 148, left), which may best 

 be compared with the leaves of Blasia, and give the prothallium an undulate 



FIG. 148. Old and full-sized prothallium of Osmunda 

 regalis, with the midrib shaded ; a antheridia, iu rhizoids, 

 v the growing-point, in the left depression of which a 

 lateral lobe is being formed (see Bot. Ztg. 1877, p. 705). 



1 Beobachtungen Uber d. Ernahrung d. Farnprothallien u. d. Vertheilung d. Sexualorgane (Bot. 

 Zeit. 1881). The same result ensues in the absence of other substances required for the nutrition of 

 the plant, or when the assimilation of carbon dioxide is impeded. Prothallia of Osmunda regalis ', 

 grown in the dark, form antheridia only according to Goppert in Sitzungsber. des intern, bot. Congr. 

 zu Petersburg, 1869. 



