290 PROCEEDINGS OF THE AMERICAN ACADEMY. 



In the majority of the forms, the (-f ) strain is characterized, in com- 

 parison with the (— ), by a greater vegetative luxuriance. In Mucor iir 

 the difference is most distinct, being markedly shown in the size of the 

 spores and in the height of the sporangial growth ; in Mucor n the dif- 

 ference is in color as well as in the height of the sporangiophores ; in 

 Mucor V, the sporangia from the (— ) strain are produced later ; in 

 M. Mucedo, tlie difference is discernible only through cultivation under 

 unfavorable conditions ; while in some others, as Rhizopus, no differences 

 in vegetative characters have yet been observed. 



In Mucor Mucedo the sexual strains have been rendered neutral in 

 their action by cultivation under unfavorable conditions, and in Phy- 

 comyces, Absidia, and Rhizopus, strains have been found which from 

 the first have shown themselves neutral in character. 



External conditions have only a secondary influence on the formation 

 of zygospores and affect the various species differently. Thus, for ex- 

 ample, while a temperature of 26°-28° C. favors zygospore production 

 in Mucor iii and Mucor N, it entirely prevents their production in 

 Mucor Mucedo, and, although a comparatively high concentration seems 

 to be necessary for the formation of the zygospores of Mucor iv, it is 

 detrimental to the production of those of Mucor n. 



In all the forms the stimulus for the development of the progametes is 

 the contact between sexually opposite hyphae. In Mucor Mucedo these 

 arise from the mycelium as differentiated zygophores, and seldom or never 

 bear sporangia ; in Absidia and Rhizopus there is no evident differen- 

 tiation into zygophoric hyphae, and apparently any of the undifferen- 

 tiated aerial hyphae may take part in the formation of zygospores ; and 

 finally in Phycomyces the progametes arise from the contact of branches 

 of the vegetative mycelium. In M. Mucedo and others of the genus 

 Mucor, a mutual attraction between the zygophores is demonstrable, but 

 in some forms, as Rhizopus, it has not been determined. 



HOMOTHALLIC FORMS. 



As has been stated in the Introduction, those species have been called 

 homothallic the zygospores of which originate from a single mycelium 

 in distinction to the heterothallic species, the zygospores of which are 

 formed by a union of gametes which have originated necessarily from 

 two different mycelia. It has been currently assumed that all sjDecies 

 belong to the first class, and consequently the distinction above given 

 has not been hitherto recognized. An enumeration, however, of those 



