GROUP III. PTERIDOPHYTA : FILICINJ] ; LEPTOSPORANGIAT.E. 403 



The arckegonium. The general description given above (p. 377) 

 of the development and structure of the archegonium, and of the 

 process of fertilisation, will suffice for this group of the Pterido- 

 phyta. It should, however, be mentioned that only a single neck- 

 canal-cell is developed. 



The root-hairs retain in all cases their typical unicellular 

 structure. They arise as tubular outgrowths from single cells, 

 having, at first, colourless walls, which eventually become 

 thickened, and assume a brown colour ; the cavity of the hair is 

 cut off by a septum from that of the cell from which it springs ; 

 their form is most commonly elongated and cylindrical, but some- 

 times (e.g. Hymenophyllacese) they are short and slightly branched. 

 The development of the root-hairs begins at the earliest stage in 

 the formation of the prothallium. Generally speaking, the root- 

 hairs are developed laterally, and as the prothalliura assumes the 

 flattened expanded form, the development extends inwards from 

 the margin, over the inferior surface, and forwards as far as the 

 posterior part of the cushion ; but in Osmunda, they are chiefly 

 developed along the middle line of the midrib, between the two 

 lateral rows of archegonia, and in Hymenophyllum they are 

 developed in isolated groups near the edge. 



The life of the gametophyte is, as a rule, short, being limited 

 by the fertilisation of an archegonium. If, however, fertilisation 

 does not take place, the prothallium continues to grow for several 

 months, or even years in the case of Osmunda. 



The gametophyte, in many instances, produces its like ; either 

 by means of adventitious branches, which become isolated and 

 then develope into prothallia ; or, by means of gemmae, which are 

 thrown off and form prothallia. Propagation by means of 

 adventitious branches has been occasionally observed in various 

 PolypodiaceaB and Cyatheacese, more especially in prothallia 

 which have remained exclusively male ; in Gleicheniaceae, 

 Osmundaceae, and Schizaeaceae (Aneimia, Mohria), the formation 

 of these branches is the rule ; it is common in Hymenophyllum. 

 In Aneimia and Mohria alone, the adventitious branches spring 

 from the the cushion ; in the other cases the branches are 

 generally developed each from a single marginal cell of the pro- 

 thallium. It has been ascertained in certain cases (e.g. Cyathea, 

 some Polypodiaceae, Aneimia) that the prothallia developed by 

 means of these adventitious branches are exclusively male. The 

 development of these branches may, it appears, be induced by 

 injury to the prothallium. 



