314 MOSSES AND FERNS CHAP. 



cell of the next phase of the prothallium's growth. The 

 divisions up to this point correspond exactly with those of 

 Aneura or Metegeria, and are also much the same as in Marat- 

 tia, except that in Onoclea the prothallium only in very rare 

 cases assumes the form of a cell mass at first. 



By the regularly alternating segments of the apical cell 

 the young prothallium soon assumes a spatulate form, which 

 becomes heart-shaped by the rapid growth of the outer cells of 

 the young segments, which grow out beyond the apical cell. 

 Sooner or later the single apical cell is replaced by two or 

 more initials formed from it in the same way as in the Marat- 

 tiace?e, and from this time on the growth is from a series of 

 marginal initials. This change is connected with the formation 

 of the thickened archegonial cushion, which, so far as I have 

 observed, does not form in Onoclea so long as the single two- 

 sided apical cell is present. 



As the prothallium grows new rhizoids grow out from the 

 marginal and ventral cells and fasten the prothallium firmly 

 to the ground. These hairs, colourless when first formed, later 

 become dark brown. 



In the genus Onoclea, as well as some other Polypodiacese, 

 the prothallia are regularly dioecious, and only a part of them 

 develop the archegonial meristem. The others remain one- 

 layered, and are often of very irregular form, and may be 

 reduced to a short row of a few cells. In Athyrium Ulix- 

 fcemina these may even be reduced to a single vegetative cell 

 besides the root-hair, and an antheridium. Cornu ( i ) records 

 similar reduced prothallia in Aspidiinn filix-mas. All of the 

 "a-meristic" prothallia, as Prantl ((4), p. 499) calls them, are 

 males. In the majority of the Polypodiacese these occur more 

 or less plentifully, and are often the result of insufficient nutri- 

 tion ; but in Onoclea it is something more than this, as not only 

 the small prothallia are male, but the large ones are exclusively 

 female, and not hermaphrodite, as in most Ferns. 



The Sex-Organs 



The first antheridia appear within three or four weeks under 

 favourable conditions, and are formed either from marginal or 

 ventral cells of the prothallium. The very young antheridium 

 is scarcely to be distinguished from a young rhizoid. Like it, 



