250 



BOTANY 



Finally a concave wall is formed above the dome-shaped one and 

 meeting it. The young antheridimn now consists of a central cell 

 enclosed by three peripheral cells, the two lower being ring-shaped, 

 the upper one somewhat lenticular. These outer cells contain chro- 

 inatophores which are absent from the central cell. 



In the lower Leptosporangiatae e.g. Osmunda, Hymenophyllum 

 there are more than three peripheral cells, and there may be a 

 special opercular cell, as there is in the Marattiacese. 



The first division of the central cell is usually vertical, and this is 

 followed by several other divisions, so that there are usually thirty- 

 two to sixty-four sperm-cells finally developed, although the number 



D 



FIG. 214. Struthiopteris Germanica. Development of archegonium. .4, longitudi- 

 nal section of apex of prothallium ; apical cell, x (X 215). B-D, archegonia 

 (X 430) ; h, neck canal-cell. 



is not always the same. Previous to the last division but two, the 

 blepharoplasts, from which the cilia are developed, make their ap- 

 pearance. 



The dehiscence of the antheridium is caused either by the rupture 

 of the cover-cell, or a small opercular cell is thrown off. The great 

 distention of the peripheral cells then forces out the separated sperm- 

 cells, whose membrane soon completely dissolves and sets free the 

 spermatozoid. In the typical Ferns the spermatozoids are relatively 

 large and consist of a spiral band, tapering at the forward end, from 

 which 'the numerous cilia extend. The larger posterior coils are 

 mainly composed of the nucleus of the sperm-cell, and enclose a deli- 

 cate vesicle containing the remains of the cytoplasm of the sperm- 



