722 U. Ndf 



DISCUSSION 



Mr. Barlow: What do you think happens in natural conditions? 

 You say that this factor is required from elsewhere, rather than the 

 prothallus itself. 



Dr. Naf: It has been demonstrated that a prothallus does not 

 begin to produce the antheridial factor at effective concentrations 

 until after it has become insensitive to it. The prothallus will there- 

 fore fail to form antheridia unless it is supplied with it from "else- 

 where," i. e., with antheridial factor secreted into the substrate by the 

 more rapidly growing and developing individuals of the gameto- 

 phyte population. I would think that this is the case also in nature. 



Mr. Barlow: This is rather curious from the biological point of 

 view, because many mechanisms are available for distributing plants 

 so they get away from each other. This would seem to be one in 

 which it is essential that two or three individuals developing from 

 the spores of the same fern should be present together before they 

 can complete their life cycle, which is rather unusual. 



Dr. Naf: It is perhaps not too rare that the completion of a life 

 cycle depends on the proximity of two or more individuals. Raper's 

 investigations on Achlya may be recalled in this connection. He 

 showed that the formation of both male and female sex organs is a 

 function of hormones produced not by the individual that forms the 

 sex organ but by individuals of the opposite sex. Raper's name also 

 recalls the myxomycetes, a quite different type of organism. The 

 work of various investigators has led to the conclusion that the aggre- 

 gation of the amoebae depends on a substance, termed acrasin, which 

 is secreted predominantly by one type of individual and is effective on 

 another. It would not be too difficult to add to these examples in 

 which the completion of a life cycle depends on interaction between 

 individuals by a chemical messenger, and therefore on proximity be- 

 tween them. 



Returning to the bracken fern, it must also be emphasized that 

 there are a number of developmental mechanisms which tend to pre- 

 vent the simultaneous occurrence of male and female gametes on the 

 same prothallus. In addition, \Vilkie has recently demonstrated that 

 this fern species is self-sterile. Thus, both the formation of the zygote 

 and of the antheridium are consequent upon the occurrence of the 

 prothalli in clusters. 



Dr. Galston: I'd like to ask two questions. Is the formation of 

 archegonia under the control of any such factors as you have de- 

 scribed or only antheridial formation? Can you tell us anything at 

 this time about the stabih'ly of the anlhcrich'uni-pronioting factor in 

 vitro? 



