Page:A Text-book of Animal Physiology.djvu/45

 mean time, we turn to the study of forms of life in many respects intermediate between plants and animals, and full of practical interest for mankind, on account of their relations to disease, as revealed by recent investigations.

PARASITIC ORGANISMS.

(Penicillium Glaucum and Mucor Mucedo). Closely related to Torula physiologically, but of more complex structure, are the molds, of which we select for convenient study the common green mold (Penicillium), found growing in dark and moist places on bread and similar substances, and the white mold (Mucor), which grows readily on manure.

The fungi originate in spores, which are essentially like Torula in structure, by a process of budding and longitudinal extension, resulting in the formation of transparent branches or tubules, filled with protoplasm and invested by cellulose walls, across which transverse partitions are found at regular intervals, and in which vacuoles are also visible.

The spores, when growing thus in a liquid, give rise to upward branches (aërial hyphæ), and downward branches or rootlets (submerged hyphæ). These multitudinous branches interlace in every direction, forming an intricate felt-work, which supports the green powder (spores) which may be so easily shaken off from a growing mold. In certain cases the aërial hyphæ terminate in tufts of branches, which, by transverse division, become split up into spores (Conidia), each of which is similar in structure to a yeast-cell.

The green coloring matter of the fungi is not chlorophyl. The Conidia germinate under the same conditions as Torula.

Mucor Mucedo.—The growth and development of this mold may be studied by simply inverting a glass tumbler over some horse-dung on a saucer, into which a very little water has been poured, and keeping the preparation in a warm place.

Very soon whitish filaments, gradually getting stronger, appear, and are finally topped by rounded heads or spore-cases (Sporangia). These filaments are the hyphæ, similar in structure to those of Penicillium. The spore-case is filled with a multitude of oval bodies (spores), resulting from the subdivision of the protoplasm, which are finally released by the