Physiology of Antheridium Formation in Ferns 



Table 1 . Number of media (out of 20 at each in- 

 terval) that have antheridium-inducing activity to the 

 indicated maximal dilutions. 



713 



Discontinuance of antheridium formation could not, therefore, 

 be ascribed to a discontinuance of antheridial factor production. 

 The alternative hypothesis suggested itself, that the maturing prothalli 

 become insensitive to antheridial factor, i.e., that they lose the ability 

 to respond with antheridium formation to the presence of that factor. 

 This hypothesis was tested first on the prothalli of Onoclea sensibilis 

 which were particularly suited to such an investigation because they 

 failed to form antheridia spontaneously under the prevailing condi- 

 tions of culture but formed them readily in response to added anther- 

 idial factor. Prothalli of this species were transferred at intervals of 

 2 days, starting 8 days after inoculation, to new medium containing 

 the active factor at a concentration series ranging from 1/2 to 1/31,250 

 full strength Pteridium medium. Table 2 shows that, whereas nearly 

 all 10-day-old prothalli were sensitive to the antheridial factor, nearly 

 all 14-day-old prothalli were insensitive to it. Subsequent investiga- 

 tions on isolated prothalli showed that the individual prothallus be- 

 came insensitive within a period of about 2 days or less. Following 

 this interval, a gametophyte fails to form antheridia even at the 

 highest concentration of the antheridial factor, i.e., a concentration 



Table 2. Number of prothalli (out of 20) that forrned antheridia 

 when transferred from basic medium to medium containing antheridial 

 factor at different intervals following inoculation of the spores. 



