Sex Determination 191 



Before leaving Pteridophytes the homosporous 

 forms, notably the true ferns, should be considered 

 briefly. In the ordinary bisexual fern prothallium, 

 antheridia and archegonia appear, as was described for 

 Riccia. A young prothallium can produce antheridia 

 only, while a mature prothallium produces archegonia 

 only. These facts are interpreted in two ways. From 

 the physiological standpoint it is claimed that more 

 nutrition is required for the production of archegonia 

 and eggs than for the production of antheridia and 

 sperms. Young prothallia, therefore, have not the 

 nutritive capacity to produce archegonia and can pro- 

 duce antheridia only. Mature prothallia, of course, have 

 a greater nutritive capacity and can produce archegonia. 

 This nutrition theory has now considerable vogue and is 

 applied not only to ferns but also to other plant groups. 



Another interpretation is given by cytologists. 

 They claim that the sexes are separated by some sex 

 chromosome mechanism at some one of the vegetative 

 divisions midway in the life of the prothallium. This 

 claim is difficult to accept on account of what it involves. 

 In accordance with this view a cell division occurs, so 

 that thereafter only archegonia will be produced; while 

 before this division only antheridia were produced. 

 It is hard to understand how a cell division can affect 

 the sex of its ancestors, the cells that came before. 

 This difficulty is avoided by saying that a sex chromo- 

 some whose presence determines maleness drops out 

 of existence at this intermediate cell division, so that 

 only archegonia could be produced thereafter. The 

 physiological claim certainly seems to be the more 

 reasonable one in this instance. 



