370 PART IV. CLASSIFICATION. 



consist of two cells, the lower of which is the stalk-cell, the upper, 

 the anther-id ial cell. The latter grows, becoming more or less 

 spherical, and undergoes repeated cell-divisions which result in 

 the formation of a wall, consisting of a single layer of cells, sur- 

 rounding a large central cell from which, by further division, the 

 mother-cells of the spermatozoids are formed. When mature, 

 absorption of water causes the rupture of the antheridium ; the 

 mother-cells of the spermatozoids are now set free, and the 

 spermatozoids soon escape from the mother-cells as coiled ciliated 

 filaments, each having usually attached to it posteriorly a vesicle 

 of granular protoplasm, the remains of the contents of the mother- 

 cell (see Fig. 222). 



The archegonium. The general description given above (p. 351) 

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

 process of fertilisation, will suffice for tnis 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 w_hich 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 prothallium 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. 



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. 



Propagation of the gametophyte by means of a gemmae is common 

 in the Hymenophyllacese, but it has also been observed in certain 

 Polypodiacese (Vittaria, Monogramme). In Hymenophyllum, the 

 gemmae are small flat plates of cells ; in species of Trichomanes, 

 Vittaria, and Monogramme, they are short spindle-shaped filaments, 

 consisting of a single row of (6-9) cells, borne on a unicellular 

 stalk or sterigma ; in Trichomanes, the gemma is attached at its 



