PHYCOMYCETES— (b) ZYGOMYCETES 417 



because in many species there is no recognisable difference of form or structure, 

 but only of function. If either be cultivated pure, and apart, the mycelium 

 bears no zygospores. But if cultures of the + and - types be started apart 

 and meet, a profuse formation of zygospores appears along the line of junction 

 (Fig. 315). The facts thus disclosed give a ready explanation of the rarity of 

 zygospores in certain cases, and their frequency in others. 



The facts of the life-history in the Zygomycetes show a less direct 

 dependence of these plants on external liquid water than in the 

 Oomycetes, for there are no zoospores motile by cilia. Still the dis- 

 semination of the spores in the Mucors is through swelling of mucilage 



Fig. 315. 

 Result of a plate-culture of the heterothallic Mucor hiemalis, made by Prof. 

 Drummond. + and — strains were started on opposite sides of the plate. The 

 dark line transversely between these shows where the cultures meet, and the zygo- 

 spores were formed. (£ natural size.) 



in water, or ejection where liquid pressure gives the propulsive power. 

 The series with branched sporangiophores, and wind-borne conidium- 

 like bodies is a step still further away from dependence upon the 

 water-medium. Comparison suggests for the more primitive sporangia 

 such as Mucor an origin from a sporangium like that of a Siphonaceous 

 Alga. The loss of motility of the spores which is involved is readily 

 understood in organisms living in moist air in place of water. 



The formation of zygospores presents the unusual condition of the 

 fusion not of single cells as in the Conjugatae, but of coenogametes 

 to form a coenozygote. There is reason to believe that numerous 

 nuclear fusions take place : in fact that the formation of the zygospore 

 is a fusion of gametangia, rather than of single gametes. If two 

 gametangia, like those of Codium or Bryopsis, were to fuse as a whole, 

 in place of opening and shedding their gametes to fuse singly, the 

 result would be very like what is seen in the Mucorales. 



If we consider the sexual reproduction of the Phycomycetes as a 

 whole, the Zygomycetes and the Oomycetes would appear to repre- 

 sent two distinct lines of evolution, the Oomycetes showing the 

 more advanced condition, in which sexual differentiation extends 

 to the gametangia themselves. 



