GROUP III. PTERIDOPHYTA. 351 



in four rows. The mature archegonium contains, in the venter, 

 the female cell (oosphere}. 



The archegonium is developed from a single superficial cell of 

 the pro thallium. This cell divides transversely into two, an upper 

 and a lower ; the former, by growth and division, forms the neck 

 of the archegonium ; the lower cell projects into the developing 

 neck, and the projecting portion becomes cut off, constituting the 

 neck-canal-cdl which sometimes divides again into two (Maratti- 

 acese, Lycopodium) ; the remainder, now termed the central cell of 

 the archegonium, divides transversely into two unequal parts, the 

 upper and smaller being the ventral canal-cell, the lower and 

 larger being the oosphere. As the archegonium becomes mature, 

 the canal-cells become mucilaginous, the neck opens by the 

 separation of the cells at the apex, and the archegonium is ready 

 for fertilisation. 



The male cell is a naked motile cell, a spermatozoid ; it is a 

 spirally coiled filament, pointed at the anterior end which bears 

 the cilia, becoming thicker towards the opposite end : the cilia 

 are numerous in Filicinse and Equisetinse ; two in Lycopodinse. 



Each spermatozoid is developed singly in a mother-cell in the 

 antheridium. The whole of the contents of the mother-cell are not, 

 however, devoted to the spermatozoid : a portion remains unused, 

 and is discharged together with the spermatozoM, to which it 

 adheres for a time as a protoplasmic vesicle containing, amongst 

 other constituents, a portion of the nuclear substance of the 

 mother-cell (see Fig. 222). 



The female cell, or oosphere, is a naked spherical cell lying in 

 the venter of the archegonium. Its development is described 

 above. 



Fertilisation is effected by the entrance of spermatozoids into 

 the open neck of the mature archegonium, and the subsequent 

 fusion of one of them with the oosphere. When, as is usually the 

 case, numerous prothallia are developed near together on the 

 ground, and become wetted by rain or dew, the ripe antheridia 

 burst and set free the spermatozoids which, swimming actively in 

 the water, are attracted to the mature archegonia by means of an 

 acid excretion which is discharged from the neck of the arche- 

 gortium when it opens. The effect of fertilisation on the oosphere 

 is that it at once surrounds itself with a cell-wall becoming the 

 oospore, and then begins to develope into the young sporophyte. 



In a few cases (e.g. species of Trichomanes and Lycopodium) the 



