GROUP III. PTERIDOPHYTA : FILICIN2E ; LEPTOSPORANGIAT^. 401 



The foregoing account refers especially to the Poly pod iacese and 

 to the Cyatheaceee : it also applies generally to the Schizeeacese, 

 though here the activity of the apical cell is of shorter duration. 

 In the Grleicheniaceee occasionally, in the Osmundaceae always, there 

 is no filamentous stage in the development of the prothallium, but 

 it at once developes into a plate or mass of cells. Moreover, m 

 the Osmundacese there is an indication of a differentiation of the 

 prothallium into shoot and root, since the first septum formed in 

 the germinating spore is transverse, dividing a posterior cell, which 

 contains relatively few chloroplastids and sometimes none, from 

 an anterior cell in which they are numerous : the posterior cell 

 grows into a simple hair which may be regarded as representing a 

 root (see p. 61), whilst the anterior cell divides to form the shoot 

 of the prothallium. In the Hymenophyllaceae the prothallium is 

 comparatively rudimentary and presents remarkable peculiarities. 

 In Hymenophyllum, the filamentous stage is either wanting or is 

 of very short duration : the prothallium is irregular in form and 

 is dichotomously branched, a rare occurrence sometimes also pre- 

 sented by old prothallia of Osmunda : it bears groups of archegonia 

 on its under surface but close to the margin, and it may consist 

 throughout of a single layer of cells, or of two or three layers 

 where the archegonia are borne. These peculiarities find their 

 parallel in certain aberrant forms among the Polypodiaceae 

 (Vittaria, Monogramme). In Trichomanes, the prothallium is 

 characteristically filamentous, consisting of a single much-branched 

 row of cells, and somewhat resembles the protonema of Mosses. 

 In some species (Trichomanes incisum and sinuosum), some of the 

 lateral branches develope into flattened cell-plates, on the margin 

 of which archegonia are borne : in others (e.g. T. pyxidiferum) 

 there are no such flattened expansions, but the cells of some of the 

 branches divide so as to form a small solid cell- mass which bears 

 archegonia. 



The gametophore. In none of these Ferns is there any special 

 organ developed to bear the antheridia, so that the gametophore 

 is always an archegoniophore. 



There are to be observed in the Ferns (as is also the case in the 

 HepaticaaJ all stages between a mere receptacle and a fully developed 

 archegoniophore. The simplest case is, perhaps, that of Hymeno- 

 phyllum, where each marginal group of archegonia may be 

 regarded as a receptacle, though the cushion is but slightly 

 developed; in the majority (most Polypodiaceae, Cyatheaceae, 



v. s. B. D D 



