BLAKESLEE. SEXUAL REPRODUCTION IN THE MUCORINEAE. 251 



and appeared only iu the upper parts of the paper cap, leaving below a 

 loose growth of pure zygospores. AVith a small amount of moisture the 

 zygospores were completely hidden below the well-developed sporaugial 

 growth, and in very dry air the formation of zygospores was entirely 

 inhibited, except in the cavities of the bread soaked with prune decoction, 

 where the amount of moisture necessary for their formation had been 

 retained. These conclusions regarding the effect of relative moisture of 

 the surrounding air upon the production of zygospores in Rhizopus have 

 been confirmed by the experience in a large number of cultures both 

 before and since the ''sexual strains" referred to later on were sepa- 

 rated. 



The early study of zygosporic cultures of lihizopus, some of the ex- 

 periments in which have been embodied in the above tables, has indicated 

 that the fundamental factor in the production of zygospores is quite inde- 

 pendent of the external conditions to which the fungus was subjected. 

 External conditions are seen to have a secondary influence, however. 

 Moisture in the surrounding air is a condition favorable to the formation 

 of zygospores and dryness is unfavorable ; while, of the nutrients tested, 

 carbohydrates are more favorable than nitrogenous compounds. In a 

 general way, therefore, the effect of external factors is found to be iu 

 agreement with the results obtained by Klebs ('98) with the homothallic 

 form Sporodinia. In the following pages the conditions which are be- 

 lieved by the writer to be of primary importance in the production of 

 zygospores in this species will be considered. 



Resolution of Rhizopus into (+) and (— ) Strains. 



In June, 1903, an attempt was made to demonstrate the existence of 

 (-f) and (— ) strains in Rhizopus by opposing a half-dozen pure transfers 

 from a zygosporic culture but without success, while in separation cul- 

 tures from the same source the zygospores were inconstant in occurrence, 

 and when present were scattered ; and on account of the stoloniferous 

 habit of growth of the fungus were prevented from showing even a sug- 

 gestion of an intermediate line such as has been above referred to as 

 occurring in such forms as Mucor Mucedo. On November 20 there were 

 in the laboratory three van Tieghem cell cultures made from zygosporic 

 hyphae, one of which was producing zygospores ; three stender dish 

 cultures which had been covered with moist filter paper caps and con- 

 tained zygospores in abundance ; and one gross zygosporic culture on 

 unsterilized bread. From these sources forty-six pure sporangial trans- 

 fers were made to paste and to potato agar each at such a distance from 



