720 U. Ndf 



then there would be little overlap between prothalli at an antheridial 

 stage and prothalli at an archegonial stage of development. Accord- 

 ingly, sexual reproduction would be hindered. 



Ameristic prothalli which form antheridia indefinitely thus appear 

 to be functional by supplying male gametes when the other two types 

 of prothalli have already attained the archegonial phase. With re- 

 gard to archegonium-forming prothalli without a prior antheridial 

 phase, the following argument seems pertinent. Antheridium forma- 

 tion diverts growth potential from the formation of vegetative cells 

 to that of antheridial cells, thus delaying the onset of the archegonial 

 phase (see section beginning on p. 712). The occurrence of arche- 

 gonium-forming prothalli without a prior antheridial phase thus 

 minimizes the time lag between the first appearance in the culture 

 of antheridium-bearing and of archegonium-bearing prothalli and 

 thereby hastens the onset of sexual reproduction. 



Several characteristics of prothallial development combine to inter- 

 fere with the simultaneous occurrence of male and female sex organs 

 on the same prothallus, especially the failure of ameristic prothalli 

 to form archegonia and the lack of a prior antheridial phase in one 

 of the two types of archegonium-forming prothalli. The more rapid 

 attainment of the archegonial phase by prothalli without a prior an- 

 theridial phase results, during an initial period of time, in the pres- 

 ence of prothalli that bear either antheridia only or archegonia 

 only. As the prothalli ivith a prior antheridial phase attain the arche- 

 gonial stage, the last-initiated antheridia do not fall apart until one 

 to three archegonia have been initiated (10). It is unlikely, though, 

 that the first-initiated archegonium matures before the last-initiated 

 antheridia have fallen apart. The simultaneous occurrence of male 

 and female gametes on the same prothallus is, however, occasionally 

 observed later because antheridia may arise on the basal outgrowths 

 which some of the archegonium-bearing gametophytes form at late 

 stages of development. 



These characteristics of prothallial development which interfere 

 Avith the simultaneous occurrence of male and female sex organs on 

 the same prothallus would seem to be functional by interfering with 

 self-fertilization. This interpretation, though, appears invalidated by 

 Wilkie's report that the prothalli of Pteridimn aqiiilinum are self- 

 sterile (15). If this finding can be confirmed, then the mentioned 

 characteristics of development could still be considered functional 

 in terms of preventing wasteful contact between incompatible ga- 

 metes. In self-fertile sjjecies, however, these characteristics of develop- 

 ment Avould clearly serve to minimize the diance of self-fertilization. 



