3IO MOSSES AND FERNS chap. 



that here 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 

 Marattiaceae, 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 root-hairs 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 Polypodiaceae, 

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

 develop the archegonial meristem. The others remain one- 

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

 reduced to a short row of a few cells. In AtJiyriuin filix- 

 fozmina these may even be reduced to a single vegetative cell 

 besides the root-hairs, and an antheridium. Cornu ^ records 

 similar reduced prothallia in Aspidiuvi jilix-nias. All of the 

 " a-meristic " prothallia, as Prantl ^ calls them, are males. In 

 the majority of the Polypodiaceae these occur more or less 

 plentifully, and are often the result of insufficient nutrition ; 

 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 first antheridia appear within three or four weeks under 

 favourable conditions, and are formed either from marginal or 

 central cells of the prothallium. The very young antheridium 

 is scarcely to be distinguished from a young root-hair. Like it, 

 it arises from a protrusion of the cell which is cut off by a wall, 

 which is usually somewhat oblique. The papilla thus formed 

 enlarges and soon becomes almost hemispherical. It contains 

 a good deal of chlorophyll and a large central nucleus surrounded 

 by dense cytoplasm. The first wall in the young antheridium 



1 Cornu (i). - Prantl, Flora, 1S78, p. 499. 



