DIV. i THALLOPHYTA 435 



which is covered with warty protuberances. As regards the behaviour of the nuclei in 

 the process of conj ugation, only a few facts are known. In Sporodinia, Phycomyces, 

 and other genera the sexual nuclei in the zygospore fuse in pairs. After a period of 

 rest the zygospore germinates, developing a germ-tube, which may at once bear a 

 sporangium (Fig. 379,5). The reduction division in Phycomyces takes place, accord- 

 ing to BUKGEFF, in the young sporangium formed on the germ-tube of the zygo- 

 spore. 



BLAKESLEE'S demonstration of the dioecious (heterothallic) nature of the 

 mycelium of most Mucorineae, for example Mucor Jlucedo and Rhizopus nigricans, is 

 of great interest. The formation of zygo- 

 spores only takes place when male and 

 female mycelia come in contact. In other 

 Mucorineae (homothallic, e.g. Sporodinia 

 grandis] the two conjugating gametes may 

 arise on the same mycelium. Exception- 

 ally in heterothallic species, such as Phyco- 

 myces nitens, a homothallic.. mycelium may 

 appear or a neutral mycelium which forms 

 sporangia only ( 57 ). 



"Within the group of the Zygomycetes 

 a reduction of sexuality can be seen. Thus, 

 in the case of certain Mucorineae, although 

 the conjugating hyphae meet in pairs, no FIG. S78.-Sporodinia grandis. Median section 

 fusion takes place, and their terminal cells of a ripe sporangium. The spores are 

 become converted directly into spores, multinucleate. ( x 425. After HARPER.) 

 which are termed AZYGOSPORES. In other 



forms, again, hyphae-producing azygospores are developed, but remain solitary, 

 and do not, as in the preceding case, come into contact with similar hyphae. 

 There are also many species in which the formation of zygospores is infrequent. 



Both the size and number of spores produced in the sporangia of Mucor Mucedo 

 are subject to variation. The sporangia of the genus Thamnidium are, on the other 

 hand, regularly dimorphic, and a large sporangium containing many spores is formed 

 at the end of the main axis of the sporangiophore, while numerous small sporangia, 

 having but few spores (sporangioles), are produced by its verticillately branching 

 lateral axes. The sporangia may at times develop only a single spore, as the 

 result of certain conditions of food-supply, and in this way assume the character of 

 conidia. This dimorphism is even more complete in the tropical genus Choanephora. 

 In this case, in addition to large sporangia, conidia are produced on special coni- 

 diophores. There are, finally, Zygomycetes (e.g. Chaetocladium) whose only 

 asexual spores are conidia. In this one group, therefore, all transitional forms, 

 from many-spored sporangia to unicellular conidia, are represented. 



Rhizopus nigricans has a poisonous substance in its cell sap which has fatal 

 effects on animals ( 58 ). 



2. The Entomophthorineae ( 59 ) is a small group of fungi which mostly live 

 parasitically iu the bodies of insects and caterpillars. The multinucleate mycelium 

 remains non-septate or later becomes divided into cells. Asexual multiplication is 

 effected by means of conidia which contain one or numerous nuclei. These arise 

 singly at the ends of branches of the mycelium and when ripe are forcibly abjected. 

 Sexual reproduction is by means of zygospores, in place of which azygospores 

 frequently arise. 



The best-known example is Empusa inuscae (Fig. 380), which is parasitic on 

 house-flies. The conidia, which are multinucleate, form a white halo around the 



