ALTERNATION OF GENERATIONS. 



475 



are borne on the under surface of the prothallium; the antheridia as little hair- 

 like structures distributed over the surface (fig. 346 % the archegonia, flask-like in 

 form, and having the expanded ventral portion sunk in the substance of the pro- 

 thallium and the neck projecting (see fig. 346 2). In the majority of Ferns both 

 sorts of sexual organ occur on the same prothallium, the archegonia on the central 

 parts and in the region of the notch or sinus, the antheridia towards the margin 

 and on the lobes. Fertilization is brought about by the escape of spirnlly twisted 



Fig. 349. — Platycerium alciconie (drawn fi-diii nature by Selleuy). 



[permatozoids from the antheridia (fig. 346 % which enter the neck of the arche- 

 l^onium, one of them fusing with the egg-cell contained in the ventral portion of 

 ihat organ (fig. 346^). We may regard the fertilized archegonium as the fruit. It 

 oes not become detached from the prothallium, but the fertilized egg-cell develops 

 n situ into the next (or asexual) generation, which differs altogether from the 

 exual one. The egg-cell divides into several cells, one of which gives rise to the 

 [oung stem, another to the first frond, a third to the primary root, whilst a fourth 

 prms a sucker or " foot ", which maintains communication for some time with the 

 Issues of the prothallium (cf. fig. 346^). Soon after the first simple frond is expanded, 

 j second is formed, and the young fern-plant is now able to continue its development 

 idependent of the prothallium. The prothallium now dies aAvay, and in its place 



