BLAKESLEE. — SEXUAL REPRODUCTION IN THE MUCORINEAE. 267" 



character associated with special varieties or races. The oily material is 

 no doubt a concentrated form of nutriment developed in connection with 

 the storing up of food in the zygospores, but wliy it should occur only 

 when certain (+) and ( — ) strains are opposed has not been investigated. 

 Tiie solution doubtless lies in a study of the differences in the individual 

 (+) and (— ) strains. 



From the spreading habit of tliis species and from the fact that certain 

 conditions of moisture and nutriment must be satisfied before zygospores 

 will form, it is rather difficult to follow the growth of the zygophoric 

 hyphae. Mutual attraction of these hyphae has been demonstrated in^ 

 certain species of the genus Mucor ; but though the same condition may 

 well hold here, this assumption is not necessary in order to account for 

 the observations which have been made. Whether this be the result of 

 accident or of some attractive force, a contact of hyphae from opposite 

 strains is the immediate stimulus for the formation of progametes, and 

 apparently this contact incites an increased branching in the filaments 

 affected, as is indicated by the fact that zygospores are found in the 

 greatest abundance on intricately branched and interwoven hyphae, 

 especially in the lower parts of the culture where sporangia do not 

 commonly occur. 



Moisture in the surrounding air has previously been shown to have 

 a decided influence on the formation of zygospores, and it is probably for 

 this reason that zygospores form preferably between the bottom and sides 

 of the culture dish and the substratum, and especially between layers of 

 filter paper placed over the nutrient for this purpose. The sporangia, 

 on the contrary, are seldom produced in similar places, but rather are 

 formed in the upper parts of the culture. It is not unusual in stender 

 dish cultures to find that the sporangia form chiefly in a thick ring 

 under the edge of the cover, and in test-tube cultures the sporangia are 

 massed in the upper part of the tube while the zj^gospores are confined 

 to the lower part. In one test-tube culture a crack was found to have 

 occurred at the base of the tube and its location was marked by a dis- 

 tinct line of sporangia which, aside from those in the upper part of the 

 tube, were confined to the limits of the crack. All these facts just men- 

 tioned point to something in the outside air which acts as a stimulus to 

 the production of sporangia. Dryness is the factor which suggests itself, 

 and which is considered by Klebs ('98) to be efi'ective in Sporodinia, but 

 test experiments have not been made for Khizopus. 



If the immediate stimulus to the formation of progametes lies in the 

 contact of hyphae from different strains, it probably becomes operative 



