PHYCOMYCETES— (b) ZYGOMYCETES 415 



and produced in larger numbers. The latter may be held as a derivative 

 condition. 



A very peculiar mode of dispersal is by forcible projection of the whole 

 sporangial head. This is seen in Pilobolus, whence its name. This fungus 

 appears with a high degree of certainty on dung kept under a bell-glass. The 

 sporangium is constructed like that of Mucor ; but at ripeness it breaks away 

 from the stalk, which has become turgid with sap under osmotic pressure. 

 By the principle of the squirt this Huid is thrown out to a distance of some 

 inches, carrying with it the sporangial head. A somewhat similar projection 

 happens in Empusa, but here it is a sporangium reduced to the state of a single 

 " conidium " that is discharged ; these conidia by adhering to any solid body, 

 cause the halo previously mentioned. There is thus a considerable variety 

 in the methods of dissemination in the Zygomycetes. 



The spores of many of the Mucorales contain more than one 

 nucleus. There are said to be two in Pilobolus, and many in Sporodinia. 

 In the latter, which grows parasitically on large saprophytic species of 

 Boletus, the sporangial head is first shut off by a septum : this becomes 

 convex, and forms the central columella. The large polynucleate 

 mass of protoplasm filling the head then undergoes cleavage into a 

 number of parts, each containing several nuclei. These round them- 

 selves off and form the spores. Such cleavage of the contents of the 

 sporangium is the typical method of formation of spores in the 

 Mucorales. The final result is the same in all : viz. germination under 

 favourable circumstances to form a new non-septate, and poly- 

 nucleate mycelium. 



The mycelium of Mucor may vary in its mode of growth according 

 to the medium. If it be submerged in fluid it shows the Oidium- 

 condition, where, dividing into short lengths, each of these may 

 increase by a process of budding not unlike a yeast. Or again, if 

 the conditions are unfavourable, the starved mycelium may divide 

 transversely, and the portions become thick-walled, as so-called 

 Chlamydospores, a state reminiscent of the behaviour of Vaucheria 

 geminata under like conditions (p. 372). When the circumstances 

 are again favourable, either of these states may pass over into the 

 normal mycelium again. 



The chief alternative mode of propagation is, however, by the 

 production of Zygospores. In many Mucorales this is normally 

 a rare event, in others common. One dominating circumstance, the 

 fact of a difference of the nature of sex, has been observed, which 

 may explain the rarity of its occurrence. The essential feature 

 is the coalescence of the ends of two equal club-shaped hyphae to 

 form a single fusion-body, which is the zygospore. Two hyphal 



B.B. 



2 D 



