the Determination of Sex in Plants. 433 



prothallia still remained without archegonia 1 . In spite of this, 

 there can be no doubt, I think, that all the prothallia are potentially 

 hermaphrodite, though, if nutrition is bad, the production of 

 archegonia remains in abeyance. No case of a purely female 

 prothallium was observed. 



Experiments were also made to test the possibility of self- 

 fertilization of a hermaphrodite prothallium. For if segregation 

 of the sex-characters takes place at the reduction division in 

 hermaphrodite races, all the gametes produced by any one pro- 

 thallium must carry the same sexual tendency 2 , and two possibilities 

 remain : 



(1) The homozygous union is impossible ; hence self-fertili- 

 zation will not take place. 



(2) If such a union is fertile, the resulting sporophyte will 

 be pure in respect of one sex, the other being completely absent. 

 The spores produced by such a plant will then give rise to pro- 

 thallia all of one and the same sex only. 



The second alternative is extremely improbable, and may, 

 I think, be at once dismissed 3 . 



The prothallia used were examined microscopically, and only 

 those were chosen which had at that time no trace of sexual 

 organs. These were washed in distilled water, were planted 

 separately in small pots containing sterilized earth. Of 34 thus 

 treated, 32 died without offspring; but in two cases a healthy 

 sporophyte was produced. The large mortality is not remarkable 

 in view of the necessity for transplanting the prothallia, many of 

 which never obtained a firm hold in the new surroundings. 

 Mr Lynch is able to confirm this observation, for he has found 

 that self-fertilization takes place in Ceratopteris. 



From the cytological examination of the prothallia it seems 

 probable that the number of chromosomes may vary in different 

 nuclei of the same prothallium, in some mitoses only 18 or 12 can 

 be made out. At the division which leads to the formation of the 

 spermatozoids 22 chromosomes are present, corresponding with 

 the number of about 44 present in the nuclei of the sporophyte, ' 

 and with the number 22 which is characteristic of the spore. 



The above results justify the conclusion that the occasional 



1 Although this experiment has yielded only a negative result, should a tendency 

 be observed for the spores of certain sporangia to produce an excess of males, it 

 would hardly be surprising ; and might indeed be expected if, by reason of the 

 position of the sporangia or through other causes, the supply of nutriment were 

 reduced. (See p. 435.) 



- Compare the results obtained by Correns ('00) and de Vries ('00) upon the 

 double fertilization in the embryo sac of Zea Mais. The two gametes of each pollen 

 grain, as also the ovum and polar nucleus of each embryo sac, are shown to carry 

 the same somatic character. 



3 Experiments are however being made to test this possibility. 



