_Oïdium albicans_ affects both animals and plants. It often attacks the mucous membrane of the mouths of young children. The spores become elongated and converted into hyphæ, and ramify about in all directions, producing a troublesome form of disease. This parasitic fungus is better known under another name, _Saccharomyces mycoderma_. Oïdium resemble algæ in their mode of life, as they are mostly found in a liquid media. The structure of all ferments is very simple: each plant is composed of a single cell, either of a spherical, elliptical, or cylindrical form, varying in size, and filled with protoplasmic and nucleated matter. This grows, and is seen to bud out and divide into two or more parts, all resembling the mother cell. Fig. 277 represents the healthy cells of yeast, _Saccharomyces cerevisiæ_, freshly taken from a brewer’s vat, and in an active stage of growth. The mode of multiplication continues as long as the plant remains in a liquid favourable to its nutrition. The changes from one stage to another are rapid, as will be noticed on reference to the consecutive formative processes the cells are known to pass through, Fig. 278 (1859). If the development of the plant is arrested by want of a saccharine or nitrogenous substance, and the liquid dries up, the protoplasm contained in the cell contracts, and the spores, or endogenous reproductive organs, of the plant will remain in a state of rest, become perfectly dry, and yet retain life. They are not easily killed, even when subjected to a very high or low temperature, they do not lose the power of germination when favourable conditions present themselves, and at once take on a new birth. There are, however, many other ferments besides that of beer-yeasts, such as alcoholic and wine ferments, the commonest of which, according to Pasteur, is _Saccharomyces ellipsoideus_. [Illustration: Fig. 278.--Development of Yeast Cells. 1. When first taken; 2. One hour after introducing a few cells into sweet-wort; 3. Three hours after; 4. Eight hours; 5. Forty-eight hours, when the cells become elongated.]