On agar and gelatin plates made so that the colonies are well isolated, the form of the latter, the rate of their growth, the character of the edge and of the surface, the elevation and the internal structure as determined by a low-power lens are often of almost diagnostic value. Also in the case of the gelatin plates, the character of the liquefaction is important. [Figs. 159 to 167] show some of the commoner characteristics to be noted.
Fig. 168.—A small mold colony natural size as viewed by transmitted light.
Fig. 169.—The same colony as viewed by reflected light.
Fig. 170.—A portion of the thin edge of the same colony as seen with the lower-power objective. × 100.
Fig. 171.—A single fruiting body (sporangium) from the same colony as seen under the lower-power objective. × 100.
Colonies of mold frequently appear on plates. These are readily differentiated from bacterial colonies after a little experience. With the naked eye usually the fine radiations of the edge of the colony are apparent. The surface appears duller and by reflected light more or less “fuzzy.” With the low-power objective the relatively large, branching threads of the mold (mycelia) show distinctly. Also the large fruiting bodies (sporangia) are easily distinguished. [Figs. 168 to 171] illustrate a common black mold (Rhizopus nigricans).