Lecture XII. 95 



close proximity to each other, short club-shaped branches arise 

 sometimes on the adjacent hyphae and grow towards each other. 

 When their ends come into contact a transverse septum develops 

 in each, cutting off the protoplasm of the slightly dilated end from 

 the rest of the branch. The walls at the point of contact become 

 dissolved and the protoplasm of the two ends mingles. By suit- 

 able stains it has been shown that there are many nuclei in the 

 ends of the two branches before they unite ; these nuclei fuse in 

 pairs from the opposite branches. The composite mass of proto- 

 plasm thus produced becomes coated over with a thick cell-wall, 

 the outer layers of which become dark and warty. The whole 

 structure is called a zygospore or zygote. Its thick wall enables it 

 to resist desiccation. After some time of dormancy its coat breaks 

 and a vertical branch emerges from it. On the apex of this a 

 sporangium is produced just like that formed on the sporangio- 

 phores of the mycelium. The spores liberated from the sporangium 

 give rise to new mycelia. 



In this sexual reproduction the two branches resemble each 

 other completely and the sexual organs are ccenocytic, and just 

 as the ccenocytic zoospores of Vaucheria appear to represent a 

 mass of undifferentiated, unicellular zoospores, so the contents of 

 these branches should be regarded as a number of undifferentiated, 

 unicellular gametes. 



While it is impossible to detect any difference between the two 

 fusing branches or the mycelia from which they come, it has been 

 found that fusion or sexual union will only take place when the 

 mycelia growing together have distinct origins. Thus the mycelia 

 developed from spores originating from the same sporangium will 

 not unite, but fusion may occur between two distinct strains culti- 

 vated together. Evidently some invisible sexual differentiation is 

 concerned. We must imagine some substance, or form of energy, 

 emitted by the hyphae which acting differentially on the two kinds 

 of mycelia, evokes the production of gametes from one ; while the 

 other under the same stimulus continues to produce vegetative 

 branches. 



Mucor grows well when supplied with proteids and carbohy- 

 drates. It is also able to nourish itself as a parasite. As a 

 general rule, however, it draws its nutrition from dead organic 

 matter and is to be classed among the saprophytes. Not having 

 any chlorophyll or other photosynthesising pigment, it is absolutely 

 necessary for it to find organic substances containing carbon. It 

 is also unable, like other plants not possessing chlorophyll, to 

 build up proteins from inorganic substances and it requires 



