FILICES 69 



anterior rows of the neck, the usual number being six, while there are 

 seldom more than four in the concave posterior side. From the middle 

 one of the three primary cells arises the whole of the axial row of cells 

 of the archegone, consisting of the central cell and the canal-cells. 

 During the development of the neck this middle cell becomes pointed 

 upwards, and forces itself between the neck-cells ; this pointed portion 

 becomes divided off by a septum, and now forms the single neck-canal- 

 cell, which lengthens as the neck lengthens. The large central cell now 

 breaks up into an upper and smaller ventral canal-cell and a much 

 larger lower cell, the protoplasmic contents of which subsequently 

 become rounded off, and constitute the oosphere. According to Campbell, 

 the ventral canal-cell is wanting in Struthiopteris germanica (L.). The 

 walls of the canal -cells swell up and become converted into mucilage, 

 and finally this thin mucilage, together with the protoplasm of the 

 canal-cells, is expelled from the open neck. The antherozoids are 

 retained by the mucilage, and collect in large numbers before the 

 archegone ; a number of them force themselves into the canal of the 

 neck, and of these some eventually reach the oosphere, and coalesce 

 with it, entering it at a light-coloured spot near the neck, which is 

 termed the receptive spot. After impregnation the neck closes up. It 

 is very rare for more than one archegone to be fertilised on the same 

 prothallium, and the enormous majority of prothallia perish without 

 producing any sporophyte generation. 



The ordinary course of the alternation of generations is occasionally 

 interrupted by apogamy or by apospory, the suppression respectively of the 

 oophyte or of the sporophyte generation. The former has been observed 

 especially in Pteris serrulata (L. fil.), the laiter in particular varieties of 

 Athyrium nlix-fcemina (Bernh.), and of Polystichum angulare (Willd.). In 

 apogamy the non-sexual fern-plant springs directly from the prothallium 

 without the intervention of a fertilised archegone. In apospory a pro- 

 thallium is produced on the surface of the frond, either in the locality 

 where the sorus would normally be found, or less often elsewhere, and 

 may assume unusual forms, sometimes that of a solid cylindrical body, 

 but bears normal archegones and antherids. Bower classifies the 

 various forms of substitutionary or correlative growths connected with 

 the suppression of the sporophyte generation under three heads, viz. 

 (i) simple prolification ; (2) sporophytic budding; (3) apospory. The 

 first hardly occurs among ferns. The second is illustrated by the 

 familiar formation of bulbils in species of Asplenium (L.), Cystopteris 

 (Bernh.), &c., in which the formation of the buds cannot be directly corre- 

 lated with arrest of spore-formation. In apospory we get a more or less 

 complete sporal arrest, but this may vary in degree. In some instances 



