426 BOTANY OF THE LIVING PLANT 



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 Chlamydospores, 

 a state reminiscent of the behaviour of Vaucheria geminata under like 

 conditions (p. 398). When the circumstances are again favourable, 

 cither 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 Mucorineae this is normally 

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

 the fact of a difference of the nature of sex, has only recently been 

 ascertained as an explanation of the rarity. 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 

 branches, either of the same mycelium (Sporodinia), or of distinct 

 mycelia (Mucor stolonifer, and other Mucors), growing towards one 

 another, meet at their apices (Fig. 345, p. 410). From the end of each 

 a conjugating cell containing protoplasm with many nuclei is cut off 

 by a transverse septum. Their apices flatten, and the cells fuse, the wall 

 separating them being absorbed. They may be called " coenogametes," 

 and the result is a " coenozygote," since many nuclei are involved 

 instead of a single one in each. The large resulting protoplast is stored 

 with nutritive material, while the outer wall thickens, often forming 

 characteristic bosses externally (Fig. 345, p. 410). In this condition the 

 zygospore may undergo a period of rest, and is resistant to unfavour- 

 able conditions. But suitable conditions induce germination, and 

 they determine what follows. Sometimes there is a formation of 

 a vegetative mycelium, sometimes an immediate formation of a 

 sporangium, as in Fig. 345, 5. 



A remarkable fact is the occurrence in some Mucorineae of " Azygo- 

 spores" that is, bodies that resemble zygospores in their structure 

 and covering, but are produced without any fusion. 



According to their power and readiness for conjugation the Mucorineae 

 have been divided into two groups. The Homothallic, which form zygospores 

 on two branches of the same mycelium, so that by sowing a single spore 

 zygospores may be obtained. This is seen in Sporodinia, in which the 

 zygospores may often be obtained in the open in autumn. Others are called 

 Heterothallic, where the presence of two individuals of two different types 

 are necessary for the production of zygospores (Rhizopus, Mucor, Phycomyces). 

 These types are distinguished as + and - , rather than as male and female. 



