Pliysiology of Antheridium Formation in Ferns 715 



marginally, others will arise also on cells inside the prothallus (three 

 of ten such antheridia are indicated by arrows). 



The area of spontaneous antheridium formation always remains 

 restricted, however. Thus, antheridia never arise in the part of the 

 prothallus anterior to the lateral meristem. Again the zone of an- 

 theridium formation in the posterior region does not stretch across 

 the whole prothallus; instead, it extends about half way or less inside 

 the prothallus from the margin of the cell plate which bears the 

 lateral meristem (zone is marked by broken line on Figure IG). The 

 other half of the posterior region remains free of antheridia except 

 for the occasional occurrence of one to three antheridia in marginal 

 cells. 



Observations following the application of the active substance to 

 cultures of various ages showed that the very young prothallus re- 

 sponded with antheridium formation throughout most of its body. 

 As the age of the prothalli increased, though, the responding area 

 gradually contracted to that described for spontaneous antheridium 

 formation. The loss of sensitivity in the anterior part of the prothal- 

 lus preceded the contraction in the responsive area of its posterior 

 region (11). 



Dopp (6) showed that the excision of the meristem from arche- 

 gonium-bearing, i.e., insensitive, prothalli of Pteridium aquilimim 

 led to the formation of antheridia in the regenerating fragments. 

 The same phenomenon was encountered in Onoclea sensihilis (au- 

 thor's results). The studies which were carried out on wings, cut 

 from insensitive, archegonium-bearing gametophytes, further showed 

 that antheridium initials in these meristemless fragments did not 

 appear, even at the highest available concentration of antheridial 

 factor, until 7 to 14 days after the wings were removed. In contrast, 

 9-day-old, i.e., still sensitive, whole prothalli will give rise to anthe- 

 ridium initials within a period of between 21/2 and 3 days after the 

 antheridial factor has been applied. Thus the removal of the meri- 

 stem leads to the restoration of sensitivity with a considerable time 

 lag. Further observation showed that antheridia were formed only 

 in those regions of the meristemless prothalli which had undergone 

 a considerable amount of cell division. This correlation between 

 antheridium formation and cell division indicates that something is 

 diluted out before the cells become sensitive again. 



As already mentioned, the application of the antheridium-induc- 

 ing factor to 9-day-old whole prothalli led to the appearance of an- 

 theridium initials within a period of between 2i/9 and 3 days at all 

 tested concentrations. If the antheridial factor was withdrawn from 

 the medium li/o days after it was applied, then no antheridia were 

 formed. 



