. Botany of the living plant. Botany. PHYCOMYCETES.—(/)) 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 tlie line of junction (Fig. 36T). 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
. Botany of the living plant. Botany. PHYCOMYCETES.—(/)) 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 tlie line of junction (Fig. 36T). 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 Hfe-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 ciha. Still the dissemina- tion of the spores in the Mucors is through swelling of mucilage in water, or ejection where fluid pressure gives the propulsive JriG. 361. Result of a plate-culture of the Iieterothallic Mucor hianalis, made by Mr. Drummond. H- 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. H 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 :
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