712 U. Ndf 



sumption the inducing molecule has undergone a gradual structural 

 modification probably concomitantly with changes in a receptor 

 molecule. The isolation and characterization of the three factors 

 should yield pertinent information. In the meantime, an attempt is 

 being made to assay for similarity between the various factors based 

 on the postulates that one factor may be a precursor of the other or 

 that one factor may behave as a chemical analogue of the other and 

 thus interfere with its synthesis or with the function it performs in 

 the initiation of antheridia. 



LOSS OF SENSITIVITY TO THE 

 ANTHERIDIUM-INDUCING FACTOR 



Many individuals in a gametophyte population of Pteridium 

 aquilinum, and of many other fern species, have an early antheridial 

 phase which is, however, terminated as the prothalli attain the arche- 

 gonial phase. Why do these prothalli discontinue the formation of 

 antheridia? The hypothesis may be proposed that they discontinue 

 forming the antheridium-inducing factor. 



It was shown, however, that the active substance becomes avail- 

 able at progressively higher concentrations as more and more pro- 

 thalli stop forming antheridia and begin to produce archegonia in- 

 stead (10). This suggests that the gametophytes continue to elaborate 

 the antheridium-inducing factor while they form archegonia. It must 

 be pointed out, however, that even mature cultures contain a small 

 percentage of gametophytes which form antheridia only throughout 

 the life of the culture (the so-called ameristic prothalli; see section 

 beginning on page 716). Accordingly, it may be these ameristic pro- 

 thalli rather than the archegonium-bearing prothalli which account 

 for the continued elaboration of the active substance by maturing 

 cultures of Pteridium aquilinum. Dopp (5) effected a separation, ne- 

 cessarily incomplete, of the two types of prothalli and found the ac- 

 tive factor to be present in the extract of either type if at somewhat 

 greater concentration in the archegonium-bearing prothalli. This 

 evidence does not unequivocally support Dopp's conclusion that 

 archegonium-bearing prothalli produce antheridial factor. Rather, 

 the active substance found in the archegonium-bearing prothalli 

 might have been produced at an earlier, antheridial stage. Moreover, 

 the active substance found in either type of prothallus might have 

 been taken up from the medium into which it was secreted by the 

 other type of prothallus. These objcc tions were met by isolating arch- 

 egonium-bearing prothalli, one per flask, and assaying the media for 

 antheridium-inducing activity at intervals over a period of time. Such 

 studies showed (see Table 1) that archegonium-bearing prothalli actu- 

 ally produced large amounts of the antheridal factor (10). 



