160 PHYCOMYCETEAE 



when two colonies of the same sex of Phycomyces nitens (Ag.) Kze. are 

 grown in a culture medium rather poor in nutritive substances, as the 

 hyphae reach a distance apart of only 1 to 1.5 mm. their further growth 

 toward one another in the medium ceases while if the two colonies are 

 of opposite sex the hyphae intermingle and soon begin to form zygospores. 

 This resembles the "barrage sexuel" discovered by Vandendries and 

 Brodie (1933) in some Basidiomyceteae between mycelia of incompatible 

 sexual phases (Chapter 12). Since the majority of Mucorales do not exhibit 

 noticeable differences in size in the uniting gametangia it is impossible, 

 for most species, to decide which plant should be called male and which 

 female. Satina and Blakeslee (1926) have made chemical tests on the two 

 sexual strains of several species and found different reactions which seem 

 to indicate to which sex each strain belongs. Blakeslee (1920) and others 

 have observed that the sexual differences show different degrees of in- 

 tensity. A plant that is very strongly male will conjugate with plants of 

 all degrees of femaleness and vice versa. On the contrary a plant weakly 

 male will not conjugate with one weakly female. Blakeslee observed at- 

 tempted conjugation between hyphae of different species or even genera, 

 but only when opposite sexes were concerned. In this way it has been 

 possible to correlate the sexes of the various genera and spe'cies of Mu- 

 corales. Burgeff (1925) actually obtained hybrids between two species 

 of Phycomyces. 



Lendner (1908) believed that of all the nuclei in the united game- 

 tangia only one pair survived to unite and become the zygote nucleus in 

 the homothallic species Sporodinia grandis Link. Miss Keene (1919) 

 studying Phycomyces nitens (Ag.) Kze., a heterothallic species, claimed 

 that the nuclei became reduced to six or eight pairs. Ling- Young (1930) 

 confirming the observations of Dangeard (1906), Moreau (1913), and 

 others believes that the majority of the nuclei undergo union instead of 

 one or a few pairs. The density of the protoplast as well as the large 

 number of granules of stored food and the thickness and hardness of the 

 zygospore wall make cytological studies of the behavior of the nuclei 

 very difficult. Cutter (1942a, b) reports the results of very extensive 

 studies which in some particulars contradict the conclusions of Lendner 

 and of Miss Keene and to a large degree substantiate those of Ling- 

 Young. He finds several types of behavior. In Mucor hiemalis Wehm., 

 Blakeslea trispora Thaxt., Ahsidia spinosa Lendn., all three heterothallic, 

 and a number of other Mucorales, all of the nuclei entering the zygote 

 unite by twos within a few days and quickly undergo meiosis, the dormant 

 zygospore containing only haploid nuclei. In Rhizopus nigricans Ehr., 

 Ahsidia glauca Hagem, and A. coerulea Bain. {Tieghemella glauca (Hagem) 

 Naum. and T. coerulea (Bain.) Naum., respectively), also heterothalHc, 

 some of the nuclei entering the zygote degenerate and some unite 



