LIFE HISTORIES OF FUNGI 



263 



they distributed that a piece of moist bread, or a jar of 

 canned fruit, left exposed for only a brief period, in almost 

 any locality, will, as noted above, soon become "moldy" 

 from the growth of mycelium produced by their germina- 

 tion. On account of the practically universal distribution 

 of ''bread mold" its spores are, of course, commonly 



Fig. iBg—Rhizoptis nigricans. A, Young sporangium, showing col- 

 umella within; B, older sporangium, with the wall removed, showing ripe 

 spores covering the columella; C, D, views of the collapsed columella after 

 dissemination of the spores. 



present in the air of laboratories, where the mold is a great 

 pest, and has therefore won the appropriate title of 

 ''laboratory weed." 



254. Sexual Reproduction. — When the hyphae of 

 mycelia derived from spores of different sex-value 

 are intermingled they frequently develop short lateral 

 branches, which grow out toward each other until their 

 tips come in contact (Fig. 190). The rich protoplasmic 

 contents at the tips are cut off from the remainder of 

 the coenocytic mycelium, the walls in contact with each 

 other become dissolved,^ and the two protoplasmic masses 

 fuse. This will be recognized as conjugation; the fusing 

 masses of protoplasm are isogametes, and the cut-off tips 

 of the conjugating branches function as gametangia. 



^ Probably by enzyme action, though this has not been actually 

 demonstrated. 



