Physiology of Antlieridium Foriuation in Ferns 717 



prior antheridial phase, archegonium-forming prothalli without a 

 prior antheridial phase, and the ameristic prothalli which produce 

 only antheridia even in mature cultures. 



The mechanism underlying the formation of ameristic prothalli 

 remains to be determined. Why do they fail to form archegonia even 

 in mature cultures? The search for the factors concerned with the 

 formation of these ameristic prothalli must also take into account 

 the striking differences between them and the archegonium-forming 

 prothalli with regard to both size and shape. Thus, the surface area 

 of ameristic prothalli amounts to less than a hundredth that of the 

 archegonium-bearing prothalli in mature cultures; again their shape 

 is highly irregular due to their diffuse growth habit while the arche- 

 gonium-bearing prothalli have the well-known heart shape that re- 

 sults from the activity of a characteristically organized meristem. 



Prantl (13) observed that the formation of ameristic prothalli is 

 favored by conditions that interfere with growth, e.g., poor mineral 

 supply, crowding, and poor lighting. He accordingly advanced the 

 hypothesis that male prothalli are gametophytes retained at a juvenile 

 stage due to adverse conditions of nutrition. This hypothesis accounts 

 at first sight for most of the distinguishing characteristics of ameristic 

 prothalli. Closer observation, though, reveals a number of discrepan- 

 cies. Thus, young prothalli pass through a fairly regular sequence of 

 cell divisions and quickly organize a growing region long before they 

 attain the size of the average ameristic prothallus. The hypothesis, 

 therefore, does not account for the diffuse growth habit and random 

 shape of ameristic prothalli. Again, Prantl's hypothesis calls for a con- 

 tinuum in sizes among the individuals of the gametophyte population. 

 While such a continuum can be readily observed in younger cultures, 

 it gives way in maturing cultures to a steadily widening hiatus between 

 the size range of ameristic prothalli, on one hand, and that of arch- 

 egonium-bearing prothalli on the other hand. 



A clue to the mechanism that underlies the formation of ameristic 

 prothalli derived from an attempt to detect such prothalli at an in- 

 cipient stage of development. Observations on young cultures of 

 Pteridium aquilinum convinced the author that prospective ameristic 

 prothalli did not begin to differ until they attained the antheridial 

 phase. At this stage of development they gave rise to antheridia not 

 only in the maturing region of the gametophyte but in the meriste- 

 matic region itself (frontal row of antheridium-bearing cells, Figure 

 IC). In contrast, the meristematic region of antheridium-bearing 

 prothalli that subsequently proceeded to form archegonia remained 

 free of antheridia at all stages of development. As indicated, such 

 incipient ameristic prothalli still had a clearly recognizable meristem 



