the haplomycelium (Fig. 12-10). In the spo- 

 rangia of the haplomycelium, the spores are 

 formed, not by meiosis as in most other 

 plants, but by mitosis — and this asexual type 

 of reproduction may continue for many gen- 

 erations. However, on rare occasions, Rhizo- 

 pus also reproduces sexually. In this case, 

 gametes are formed on two neighboring 

 mycelia, and these fuse to form a diploid 

 zygospore (Fig. 12-10). This zygote is very 

 resistant to unfavorable conditions. But 

 when a favorable environment is found, the 

 zygospore gives rise by mitosis to a new 

 mycelium, called the diplomycelium, be- 



Reproduction in Multicellular Plants -211 



cause all the nuclei are diploid. The diplo- 

 mycelium soon develops its sporangium, 

 which contains diploid spore mother cells, or 

 sporocytes. In the sporangium of the diplo- 

 mycelium (Fig. 12-10), the spores are formed 

 by meiosis, and after liberation, each haploid 

 spore gives rise to a new haplomycelium. 



The haplomycelium, since it is haploid 

 and forms gametes, is equivalent to a game- 

 tophyte generation; and the diplomycelium, 

 since it is diploid and reproduces by sporula- 

 tion, represents the sporophyte. However, the 

 cycle in Rhizopus is irregular, since fertiliza- 

 tion occurs only rarely, and since the game- 



SPORES , LIBERATED 

 RIPE SPORANGIA 



IMMATURE SPORANGIUM. 



SPORANGIOPHORE 

 GERMINATING SPORE 

 STOLONS 

 BREAD SURFACE 



RHIZOIDS 



MOLD ON BREAD 

 PROGAMETES 

 SUSPENSOR 

 GAMETES 



Fig. 12-10. Reproductive stages in the common bread mold (Rhizopus nigricans). The 

 ordinary bread mold (left of figure) possesses haploid cells, and this haplomycelium 

 perpetuates itself asexually by forming successive generations of haploid spores by 

 mitosis from haploid spore mother cells in the sporangia. On rare occasions, however, 

 two different (called plus and minus) strains of the mold participate in sexual reproduc- 

 tion. Then neighboring aerial hyphae develop gametes. These fuse to form a zygote, 

 which soon metamorphoses into a heavily coated zygospore. The diploid stage of the 

 bread mold is short-lived, however. The zygospore, when it germinates, gives rise to the 

 highly simplified diplomycelium, consisting of a single sporangiophore, with just one 

 sporangium at the apex. In this sporangium, the diploid spore mother cells undergo 

 meiosis, giving rise to haploid spores, which initiate the haplomycelia of the next 

 generation. 



