238 MUCOR 



bursting of the sporangium. It is also responsible for the adhesive 

 nature of the spores, which are so widely disseminated by the 

 wind that they are almost ubiquitous. They are extremely 

 resistant, and are capable of remaining dormant for long periods. 

 On germination they grow direct into a new plant without the 

 production of zoospores. 



Under certain circumstances (e.g. growth in a liquid) the 

 hyphse of M ucor become septate, the protoplasts of the individual 

 compartments contract, and secrete a thick dark-coloured wall. 

 The special resting spores thus formed are known as chlamydo- 

 spores, which likewise produce new plants direct. 



Sexual reproduction is rare, since in most species it only 

 takes place between two mycelia belonging to distinct strains. 

 The difference is, however, purely physiological, since the two 

 are not distinguishable outwardly. The gametes are produced 

 within swollen club-shaped branches whose end-portions become 

 separated off by cross-walls (Fig. 125, d, g.) . Two perfectly similar 

 branches meet by their tips, one being derived from each of the 

 two plants (Fig. 125, d), and the intervening membrane breaks 

 down, whereupon fusion of the protoplasts and of their nuclei 

 ensues (e). The product, deriving nutriment from the respective 

 mycelia, subsequently undergoes slow enlargement to form a 

 large spherical zygospore with a thick black wall (Fig. 125, /). 

 Germination is direct, as in the case of the spores. 



The sexual process of Mucor is thus isogamous and analogous 

 to that of the Conjugate, where likewise an entire plant is often 

 of one sex or the other (cf. p. 227). In the parasitic Fungus 

 which attacks house-flies (Empusa musccz) the fusing cells are, 

 however, of unequal size, and thus show a structural distinction. 



Mucor and Empusa belong to a subdivision of; the Phyco- 

 mycetes, known as the Zygomycetes, all characterised by this 

 type of sexual process. The other forms previously considered 

 are oogamous and classed as Oomycetes, but they are also dis- 

 tinguished by the ease with whiclTtKey produce zoospores. The 

 Zygomycetes are much more markedly adapted to terrestrial 

 conditions from this point of view, since all of them reproduce 

 by means of motionless spores. 



The second great group of Fungi, the Ascomycetes, are char- 

 acterised by their method of spore-formation and by the absence 



