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PLANT MORPHOLOGY 



furrows that start at the surface and grow inward (Fig. 9ZC-E) . Finally, 

 each protoplast secretes a cell wall and becomes a minute, black, multi- 

 nucleate spore (Fig. 93F). The spores, produced in enormous numbers, 

 are liberated into the air by rupture of the sporangial wall. Upon reach- 

 ing a suitable supply of food, they give rise to new mycelia. The replace- 

 ment of zoospores by aerial spores is a notable feature of the Mucorales. 



Fig. 92. Rhizopus nigricans. A, horizontal branch of mycelium producing haustoria and 

 sporangia, X 15; B, a mature sporangium, showing central columella, X 150; C, D, E, stages 

 in sexual reproduction, resulting in the formation of a heavy-walled zygote, X 150. 



Sexual reproduction occurs in Rhizopus only under special conditions 

 (Fig. 92C-E'). A short lateral branch is put out by each of two hyphae 

 lying parallel to each other. Their tips come in contact, enlarge, and 

 from each a multinucleate cell is cut off by a cell wall. Although ordi- 

 narily of the same size, often one cell is slightly larger than the other. 

 Finally, the wall between the cells is dissolved and their contents fuse to 

 form a zygote. Many of the nuclei become associated in pairs and fuse, 

 the others disintegrating. The zygote enlarges and becomes a thick- 

 walled resting cell. The two conjugating cells are usually interpreted as 

 gametangia and their contents as large compound isogametes. It has 

 been observed in other Mucorales, but not in Rhizopus, that the zygote. 



