306 PLANT GROWTH SUBSTANCES 



tvveen the two compatible mycelia. Corraloid swellings, the zygophores 

 or sexual organ primordia, formed on the ( — ) mycelium opposite new 

 growth across the membrane from the agar block containing the (+) 

 hyphae. Thus activation without contact at a distance, or telomorphosis, 

 was proven. Also in a number of cases compatible zygophores on opposite 

 sides of the membrane were seen to grow towards each other; this he 

 termed zygotropism. Thus the initiation of the formation of sexual organs 

 and their correlated development were proved to be the result of 

 hormonal action. 



These results were confirmed and in some cases extended by a number 

 of workers on M. mucedo and other heterothallic members of the 

 Mucorales. Verkaik (69) claimed that only the (+) strain could be 

 induced to initiate progametangia in membrane matings and that there- 

 fore only the ( — ) strain secreted a stimulatory substance. Ronsdorf (60) 

 and Kohler (28), working with P. B la J^esleeanus and M. mucedo respec- 

 tively, found that zygophores could be induced on the mycelia of both 

 strains and that at least two substances must be involved; one from each 

 strain acting upon the mycelium of the opposite and compatible strain. 

 The former worker attempted unsuccessfully to extract the active 

 substances from the mycelia of both strains. The activity of histamine 

 in augmenting the sexual reaction intensity in matings of (+) and (— ) 

 strains was interpreted as indicating a close chemical relationship be- 

 tween this substance and the sexual hormones secreted by the plants. 



Krafczyk (29,30), in experimental work on Pilobolus crystallinus, 

 postulated much more extensive activity of sexual hormones than had 

 been claimed by previous workers. The mutual attraction of vegetative 

 hyphae of the two sexual strains before the formation of zygophores, the 

 initiation of sexual organ primordia, the coordinated enlargement of the 

 progametangia while in contact at their tips, and the delimitation of the 

 gametangia by septation of the progametangia he considered to be under 

 hormonal control, that is, hormonal control of the entire sexual process 

 until the time of gametangial differentiation. 



In none of the Mucors, however, has sufficient evidence been obtained 

 to postulate either the number of substances involved, the loci of their 

 origins, or their specific activities in correlating the various phases of the 

 sexual reproductive process. 



In 193 1 a paper by Moreau and Moruzi (38) touched off an interesting 

 controversy. These workers claimed the production of perithecia as the 



