We are now in a position to consider particular fermentations and their causal micro-organisms. These latter are of various kinds, belonging, according to botanical classification, to various different subdivisions of the non-flowering portion of the vegetable kingdom. A large part of fermentation is based upon the growth of a class of microscopic plants termed _yeasts_. These differ from the bacteria in but few particulars, mainly in their method of reproduction by budding (instead of dividing or sporulating, like the bacteria). Their chemical action is closely allied to that of the bacteria. Secondly, there are special fermentations and modifications of yeast fermentation due to _bacteria_. Thirdly, a group of somewhat more highly specialised vegetable cells, known as _moulds_, make a perceptible contribution in this direction. According to Hansen, these latter, so far as they are really alcoholic ferments, induce fermentation, not only in solutions of dextrose and invert sugar, but also in solutions of maltose. _Mucor racemosus_ is the only member that is capable of inverting a cane-sugar solution; _M. erectus_ is the most active fermenter, yielding eight per cent. by volume of alcohol in ordinary beer wort. Each of these will be referred to as they occur in considering the five important fermentations already mentioned. [Illustration: SACCHAROMYCES CEREVISIÆ] The general microscopic appearance of yeast cells may be shortly stated as follows: they are round or oval cells, and by budding become daughter yeasts. Each consists of a membrane and clear homogeneous contents. As they perform their function of fermentation, vacuoles, fat-globules, and other granules make their appearance in the enclosed plasma. As in many vegetable cells a _nucleus_ was detected by Schmitz by means of special methods of staining, Hansen has found the nucleus in old yeast cells from "films" without any special staining. 1. _Alcoholic Fermentation._ Cause, yeast; medium, sugar solutions; result, alcohol and carbonic acid. It was Caignard-Latour who first demonstrated that yeast cells, by their growth and multiplication, set up a chemical change in sugar solutions which resulted in the transference of the oxygen from the hydrogen in the sugar compound to the carbon atoms, that is to say, in the evolution of carbonic acid gas and the production, as a result, of alcohol. If we were to express this in a chemical formula, it would read as follows: C_{6}H_{12}O_{6} (plus the yeast) = 2 C_{2}H_{6}O + 2 CO_{2}.