PTERIDOPHYTA 



251 



cell. This may become much distended, and often contains small 

 granules of starch. 



Archegonium. In the typical Ferns the archegoniuui (Fig. 214) 

 is formed upon the lower surface of the thickened cushion back of 

 the apex. In Botrychium it is borne upon the upper surface of the 

 subterranean gametophyte, and in Osmunda the archegonia forms 

 two rows along the sides of the midrib. The mother-cell divides 

 into two by a transverse wall, and the inner cell usually divides 

 again into an inner or basal cell, and a central cell from which the 

 egg-cell and canal-cells are formed. The outer cell, which corre- 

 sponds to the terminal or cover-cell of the Liverwort archegonium, 

 divides by cross-walls into four cells, which, by a series of transverse 

 divisions, give rise to the four-rowed neck characteristic of the 



B 



FIG. 215. A, Osmunda cinnamomea, section of recently fertilized archegonium 

 (X 450). A spermatozoid has penetrated the nucleus of the egg, and several are in 

 the space ahove the egg. B, Onoclea sensibifis. Egg fourteen hours after the 

 penetration of the spermatozoid, which is still recognizable within the egg-nucleus 

 (X900). (B, after SHAW.) 



Pteridophytes. In the Polypodiaceae the two posterior rows remain 

 shorter than the anterior ones, and the neck is curved backward, 

 probably an adaptation for facilitating the entrance of the sperm a- 

 tozoids. In the lower Leptosporangiatae, and all the Eusporangiatse, 

 the neck is straight. The base of the archegonium is always coherent 

 with the surrounding tissue, as in the Anthocerotales. 



The middle cell of the original three becomes pointed above, and 

 this portion is cut off as the neck canal-cell, which subsequently 

 divides more or less completely into two. A second transverse, or 

 concave division-wall, cuts off the ventral canal-cell from the egg, 

 which later contracts so as not to fill the cavity of the venter. The 

 walls of the canal-cells, as in the Bryophytes, become mucilaginous, 

 and effect the opening of the ripe archegonium, when water is 



