PHYCOMYCETES. (b) ZYGOMYCETES 427 



because 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. 361). The behaviour is thus similar to that seen in unisexual plants. 

 Functionally the Homothallic may be compared with the Monoecious condition, 

 the Heterothallic with the Dioecious state. But these comparisons must 

 not be pressed too closely. 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 fluid water than in the Oomy- 

 cetes, for there are no zoospores motile by cilia. Still the dissemina- 

 tion of the spores in the Mucbrs is through swelling of mucilage in 

 water, or ejection where fluid pressure gives the propulsive power. 



FIG. 361. 



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

 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, (i natural size.) 



The series with branched sporangiophores, and wind-borne conidia 

 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 under- 

 stood 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 coeno- 

 gametes to form a coeno-zygote. 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 Mucorineae. 

 Provisionally this seems the best interpretation of the facts. Even 

 the Azygospores may receive a similar interpretation. In some 



