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 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. 
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 ccenocytic mycelium, the walls in contact with each 
other become dissolved, 1 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. 
1 Probably by enzyme action, though this has not been actually 
demonstrated. 
