LIFE HISTORIES OF FUNGI 263 



millions. So widely and so thickly are 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 germination. On 

 account of the practically universal distribution of "bread 



Fig. 189. — Rhizopus nigricans. A, Young sporangium, showing col- 

 umella witMn; 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. 



mold " its spores are, of course, commonly 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 hyphse 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. 



