718 U. Ndf 



and their shape conformed to that of other individuals at a compa- 

 rable stage of development. The diffuse growth habit and random 

 shape of ameristic prothalli is thus acquired subsequently. 



The questions arose: AVhat are the conditions that favor the 

 formation of antheridia by meristematic cells, and can such anther- 

 idium formation account for the distinctive characteristics of amer- 

 istic prothalli? 



The described incipient ameristic prothalli were observed to be 

 among the smallest individuals of the gametophyte population. The 

 hypothesis may thus be proposed that antheridium formation by 

 meristematic cells is favored by conditions of slow growth. 



It was demonstrated that all individuals of the gametophyte pop- 

 ulation will give rise to antheridia in their meristematic cells and 

 subsequently acquire the characteristics of ameristic prothalli if, 

 firstly, they are cultivated under conditions of slow growth and if, 

 secondly, they are all supplied with antheridial factor while they are 

 still sensitive to it. The requirement for added antheridial factor is 

 understood if it is recalled, firstly, that the formation of ameristic 

 prothalli is related to antheridium formation (in the meristematic 

 region) and, secondly, that the most rapidly developing individuals 

 (archegonium-forming prothalli without a prior antheridial phase) 

 are not otherwise exposed to antheridial factor during their sensitive 

 period. 



The question as to how slow growth favors the formation of an- 

 theridia by meristematic cells has not as yet been answered. It may 

 be pointed out, however, that the meristematic cells of rapidly grow- 

 ing prothalli divide at a rate of at least one per day. On the other 

 hand, the appearance of antheridium initials follows the application 

 of antheridial factor with a delay of between 21/2 and 3 days. It is 

 possible, therefore, that the meristematic cells of rapidly growing 

 prothalli remain free of antheridia because they divide before the 

 induction of an antheridium can take effect. 



It was emphasized above that incipient ameristic prothalli are of 

 no more irregular shape than archegonium-forming prothalli at a 

 comparable stage of development. The formation of antheridia in 

 the meristematic region is, however, soon followed by a breakdown 

 in meristematic growth. Ihe meristem ceases to operate as a func- 

 tional unit and cell division resumes in the basal region of the game- 

 tophyte. The formation of antheridia in the meristematic region 

 thus releases the potentiality of maturing cells for cell division which 

 is suppressed in the presence of an actively functioning (antheridium- 

 free) meristem. Antheridium formation by meristematic cells thus 

 brings forth the same response as the excision of the meristem itself (1). 

 The assumption of heart shape is preceded by, and dependent on, 



