ALTERNATION OF GENERATIONS. 



475 



are borne on the under surface of the prothallium; the antheridia as httle 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 ^). 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 spirally twisted 



Fig. Si9.—Platycerium alcicorne (drawn from nature by Sellenj). 



spermatozoids from the antheridia (fig. 346 *), which enter the neck of the arche- 

 gonium, one of them fusing with the egg-cell contained in the ventral portion of 

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

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

 in situ into the next (or asexual) generation, which difiers altogether from the 

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

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

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

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

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

 independent of the prothallium. The prothallium now dies away, and in its place 



