714 



U. Ndf 



Days a'ter inoculation of the spores 



Fig. 2. Sequence of developmental stages in prothalli of Onoclea seiisibilis. (a) Num- 

 bers of prothalli (out of 40) that have attained heart shape, (b) Numbers of 

 prothalli (out of 40) that have become insensitive to the antheridial factor, (c) Num- 

 bers of prothalli (out of 10) that have initiated one or more archegonia. 



15,000 times higher than that sufficient to induce antheridia in pro- 

 thalli just 2 days younger. 



Figure 2 relates the onset of this loss of sensitivity to two other 

 major events in the development of Oiioclea gametophytes: attain- 

 ment of heart shape and of the archegonial phase. The results re- 

 corded in this figure show that the prothalli became insensitive to 

 the antheridial factor about 2 days after attaining heart shape and 

 6 to 7 days prior to the attainment of the archegonial phase. 



In cultures of Pteridium aquiUniim the first insensitive prothalli 

 were observed in 7-day-old cultures. The studies on this fern species 

 further showed that the organization of the meristem, the activity 

 of which results in the attainment of heart shape, again precedes the 

 loss of sensitivity to the antheridial factor (10). 



Similar studies were carried out on Anejnia pJiylUtidis. Figures IF 

 atul IG show that the prothalli of this fern species have a lateral 

 meristem, a phenomenon encountered in only few fern species. The 

 first antheridium is invariably initiated by a marginal cell a short 

 distance back of the meristematic initial. Marginal cells cut off 

 subsc(|ucntly by the meristematic initial also give rise to antheridia 

 so that ultimately a whole row of them can be seen (marked h in Fig- 

 ure IG). In contrast, the marginal cells given off by the initial toward 

 the tip of the prothallus fail to form antheridia even in the presence 

 of added Anemia factor. Instead, some of them give rise to hook- 

 shaped hairs (arrow r). Once the first antheridium has been initiated 



