BLAKESLEE. — SEXUAL REPRODUCTION IN THE MUCORINEAE. 217 



tial, since he has cultivated a mycelium for four weeks on an unlimited 

 substratum with only nonsexual reproduction. The sexual reproduction 

 is not necessarily induced after a series of nonsexual generations, since he 

 has cultivated Rhizopus, Mucor Mucedo, and M. racemosus for four years 

 up to nearly one hundred nonsexual generations. With Sporodinia and 

 Spinellus no culture fails to show zygospores; with Mortierella Rosta- 

 finskii, Piptocephalis, and Chaetocladium, zygospores form after a few 

 generations of nonsexual reproduction; while with Rhizopus and J/. 

 Mucedo a longer time elapses before the appearance of zygospores. 

 There is thus, according to Brefeld, a gradual withdrawal of the sexual 

 in favor of the nonsexual reproduction, till in such forms as M. racemosus 

 and Pilobolus this has resulted almost or entirely in the disappearance 

 of sexuality. Brefeld's ('00 and '01) final conclusion is that in most 

 Zygomycetes the zygospore formation is dependent on unknown inner 

 causes, and it is only by accident that it may be found. 



While the authors thus far cited have based their theories and conclu- 

 sions chiefly or wholly on what they could learn from the chance occur- 

 rence of zygospores in their cultures, and scarcely ever resorted to 

 experiments, more recent investigators have endeavored to subject their 

 processes to a more exact experimental research. Klebs ('98) was the 

 first to investigate in detail the effect of varying external conditions on 

 the resulting form of reproduction of any of the Mucoriueae, and in his 

 paper on Sporodinia we have a careful account of the factors influencing 

 sporangium and zygospore formation in this homothallic form. Van 

 Tieghem's idea that lack of oxygen' is the cause for zygospore formation 

 is disproved, and it is shown that all the influences which hinder sporan- 

 gium formation, as a reduction in the amount or in the concentration of 

 the nutrient, hinder much more the formation of zygospores. Most of 

 the carbohydrates tested with gelatine and the acid salts of organic acids 

 favor zygospores, while the nitrogen compounds are unfavorable. The 

 immediate cause of sporangium formation is held to be transpiration from 

 the hyphae, and though impoverishment of nutriment has a similar 

 effect it is impossible to obtain zygospores in relatively dry air. The 

 stimulus for production of zygospores, however, is conceived to be the de- 

 creased transpiration due to increased relative humidity in the air. The 

 limits of concentration within which zygospores are produced vary greatly, 

 but for grape sugar in gelatine, lie between one per cent and fifty per 

 cent. The water content of the substratum, temperature, and light, 

 are effective only as they influence the relative humidity and consequent 

 transpiration. The formation of azygospores is said to be induced in 



