FUNGI, SEX IN MUCORS 253 



natural conditions these two strains or sets of mycelia of opposite 

 nature are sometimes growing mixed together, but more often, 

 probably, the two strains are growing separately. If one obtains 

 a culture of only one strain on the bread no zygospores will be 

 produced. But if the mixed strains are obtained, then zygospores 

 will be produced in number down in the lower part of the vessel 

 where it is moist, usually in small spaces between the paper and 

 the wall of the vessel or bread. It will be remembered that the 

 plant body of an alga or a fungus is called a thallus. The mycelium 

 from a single spore, then, is a thallus. Since two different thalli 

 of the bread mold are necessary to bring about conjugation and 

 produce zygospores, such a thallus plant is said to be heterothallic, 

 because other or different thalli must be brought together for 

 sexual reproduction. They are also said to be dioecious. The 

 Mucor mucedo above mentioned is also dioecious. In some of the 

 molds there is a difference in the size of the two different thalli; 

 the one supposed to represent the female is larger and is by some 

 indicated as + . The other being smaller is indicated by . By 

 extension + and are applied to corresponding strains in species 

 where the thalli are of equal size. 



408. Monoecious Mucors.* Some of the mucors are monoe- 

 cious,! i-C-j both sexes are present in the mycelium from a 

 single spore. This is true in the mushroom mold (Sporodinia 

 grandis), a common mold growing on decaying mushrooms in 

 the woods. 



409. Nature of the spores in the germ sporangium. The 

 name germ sporangium is by some applied to the spore case which 

 is formed from the germinating zygospore. Since the sexes 

 become mixed in the zygospore of the dioecious species they must 

 be separated again, otherwise the species would become monoecious. 

 This separation takes place in Phycomyces nitens in the formation 

 of the spores in the -germ sporangia, so that there are + and 

 spores mixed in a single spore case. In Mucor mucedo they are 



* Mucorinese, or Mucorales. The order containing different genera and 

 many species of Mucors is called Mucorinece, or Mucorales. 

 t Homothallic, because all the thalli are alike sexually. 



