ORDER MUCORALES 



157 



Fig. 53. Miicorales. (A-B) Family Mucoraceae. Zygorhynchus macrosporus Ling- 

 Young. (A) Sporangiophores and sporangia. (B) Anisogamous formation of zygo- 

 spores. (C-K) Family Thamnidiaceae. (C-J) Dicranophorafulva Schroet. (C) Primary 

 sporangium and clusters of sporangioles. (D-J) Successive stages in sexual reproduc- 

 tion. (K) Thamnidium elegans Link, sporangiophore showing large terminal sporan- 

 gium and lateral clusters of sporangioles. (A-B, after Ling- Young: Rev. gin. botan., 

 42(495) :152. C-J, after Dobbs, Brit. Mycol. Soc. Trans., 21(1-2) :172, 183. K, after 

 Brefeld from Comparative Morphology of Fungi, by Gaumann and Dodge. McGraw- 

 Hill Book Company.) 



nated from plants obtained in various places, it is found that sometimes 

 at the line of contact between two cultures very abundant zygospore 

 formation occurs. On their part these other plants are self-sterile or 

 sterile when grown in contact with each other so far as sexual reproduc- 

 tion is concerned. Thus A. F. Blakeslee (1904) determined that for many 

 species of Mucorales there are two sexes, each capable of almost indefinite 

 perpetuation by means of the asexual spores, but producing zygospores 

 only when the mycelium of one sex comes into contact with that of the 

 other sex. He named this phenomenon heterothallism and called such 

 molds heterothalHc. In contrast to these he found many species in which, 

 when the proper conditions of environment and nutrition were met, zygo- 

 spore production would occur within the mycelium, originating from a 

 single spore. Such molds he called homothallic. The common bread mold, 

 usually called Rhizopus nigricans Ehr., is a good example of a hetero- 

 thallic mold while Sporodinia grandis Link, a mold frequently found on 

 decaying mushrooms, is homothallic. Ling- Young (1930) showed that 



