Darling : Sex in dioecious plants 179 



These experiments seemed to them to indicate that the pro- 

 tonemata which are produced by the spores or by the tissue of the 

 gametophyte are unisexual. However, either one of two interpre- 

 tations may be given to the results : (i) The protonemata which 

 are apparently unisexual contain the characters of both sexes, but 

 one set has become latent to the extent that it does not become 



* 



active by artificially varying the conditions of growth. (2) The 

 characters of but one sex are really present. This latter interpre- 



r 



tation is the one given by the Marchals. 



A similar case in the Bryophyta, that of Marchantia poly- 

 morpha, has been worked out, although with much less certainty. 

 It is reported by Blakeslee ('06) that the sporophyte produces two 

 kinds of jspores in the same sporangium, one giving rise to male 

 thalli, the other to female. The gametophytes appear to be uni- 

 sexual, as shown by the work o^ Noll, in which he cultivated . 

 plants of both sexes by means of the gemmae for over thirty 

 generations but was unable to chan^ge the sex character of the 

 thalli by growing them under varied conditions. 



Blakeslee ('06) finds a similar condition existing in Mucor 

 Mucedo. He states that two kinds of spores are produced in a 

 single sporangium, the one giving rise to unisexual male mycelium, 

 the other to unisexual female. He finds that the zygospores are 

 bisexual. Although neither in Marchantia nor in Mtuor were the 

 proportions of the two kinds of spores obtained, yet as far as the 

 observations go they seem to indicate that the same general phe- 

 nomenon obtains as was found by the Marchals in the dioecious 



mosses. 



I have been unable to find any account of work done on the 



Pteridophyta that would indicate the occurrence of two kinds of 

 spores in the same sporangium. Prantl ('81) and Duval-Jouve 

 ('63) working on the ferns and equisetums, respectively, both state 

 that there is a tendency to dioeciousness but that occasionally 

 both sex organs may be found on a single prothaHium ; Stras- 

 burger ('00) also believes that dioeciousness has not been full 

 established in the ferns. Wherever dioeciousness occurs in t 

 Pteridophyta, it is apparently associated with heterospory, and it 

 must follow that in these cases the separation of the two ten- 

 dencies occurs previous to spore formation: The difference then 



