Physiology of Antheridium Formation in Ferns 719 



a change in the organization of the meristem. In turn, the attainment 

 of heart shape invariably precedes the loss of sensitivity to the anther- 

 idial factor and the attainment of the archegonial phase (10). The 

 failure of ameristic prothalli to assume heart shape (and their re- 

 sulting diffuse growth habit), their failure to become insensitive 

 to the antheridial factor and to form archegonia may thus be traced 

 to the breakdown in meristematic growth at an early stage of develop- 

 ment. The wide hiatus in sizes between the ameristic and the arche- 

 gonium-bearing prothalli of mature cultures remains to be explained. 

 Clearly, the observation that ameristic prothalli arise from the most 

 slowly growing individuals cannot account for this hiatus. 



Antheridium formation entails a diversion of growth potential 

 from the formation of vegetative cells to that of antheridial cells (9). 

 Also, it has long been known that a vegetative cell bears not infre- 

 quently two or even three antheridia, mostly at different stages of 

 development. A vegetative cell might thus give rise to new antheridia 

 repeatedly after the previously formed, ephemeral structures have 

 fallen apart. The resulting diversion of growth potential from the 

 formation of vegetative cells to that of antheridial cells could fully 

 account for the ever widening hiatus between the sizes of ameristic 

 prothalli which form antheridia even in mature cultures and the 

 archegonium-forming prothalli which either lack a prior antheridial 

 phase or discontinue it at an early stage of development. 



It was stressed above that the antheridia a prothallus forms are 

 initiated in response to antheridial factor produced by more rapidly 

 developing individuals of the gametophyte population which them- 

 selves have already become insensitive to it. The ameristic prothalli, 

 like the archegonium-forming prothalli with a prior antheridial 

 phase, thus are the result of interaction between gametophytes. The 

 above results are further in agreement with the postulate that the 

 several distinctive characteristics of ameristic prothalli are all the con- 

 sequence of antheridium formation (in the meristematic region). 

 These distinctive characteristics should therefore disappear if the pro- 

 thalli are removed from the interaction with other individuals of the 

 gametophyte population. 



Ameristic prothalli taken from mature cultures actually gave rise 

 to heart-shaped lobes, became insensitive to the antheridial factor, 

 and attained the archegonial phase after they were washed and trans- 

 ferred, one to a flask, to new medium. 



ON THE PHYSIOLOGY OF REPRODUCTION IN 

 PTERIDIUM AQUILINVM 



It should be of interest to consider the question as to how these 

 three types of prothalli relate to sexual reproduction. If in all pro- 

 thalli an antheridial phase were followed by an archegonial phase, 



