a sexuality quite comparable to that which is observed in the lower Ascomycetes. The yeasts make up a family of the Ascomycetes known as Saccharomycetes.
True yeasts never produce endo- or ascospores. Do they represent forms derived from more highly developed fungi and made constant by a long adaptation to this condition? Or, are they true Saccharomycetes having lost, by a series of unknown circumstances, their aptitude of forming spores? We shall see that a definite loss of this characteristic has often been proved among the Saccharomycetes during certain special conditions. Be that as it may, the origin of the yeasts is entirely ignored; the Saccharomycetes are then separated and regarded as yeasts of uncertain origin. In this book, we shall examine extendedly only the true yeasts; first, yeasts with ascospores, or Saccharomycetes; secondly, those cells which exhibit all the characteristics of Saccharomycetes with the exception of ascospore formation and which, from the above, one might call pseudo-yeasts. All of those yeast-like structures of other fungi will be neglected. True yeasts are very abundant in nature; over five hundred are known. The limits of our study must be rather wide.
The study of yeasts is intimately associated with that of fermentation. The idea that alcoholic fermentations are caused by living organisms originated with Linne. In 1680 Leewenhoek first described the yeasts. He described them as globular bodies, oval or spherical in shape. In 1799 Fabroni compared yeasts to albuminoids. About 1825, and for some time after this, Mitscherlick, Cagnard-Latour, Schwann, and Ktitzing demonstrated that beer and wine yeasts were composed of cells which multiplied by budding. In 1839 Schwann observed, for the first time, endospores in yeasts. He proved that they might be freed by a rupturing of the cell wall.
But the nature of yeasts has been definitely known since the period in which Pasteur commenced his investigations on fermentation. Up to this time, it was known that beer yeast multiplied when introduced into saccharine wort; it was believed that it was formed spontaneously and that, in the yeast, was an occult force which produced the fermentation; that was all there was to it. With Pasteur, definite knowledge with regard to yeasts commenced. It was in 1859 that he established, by his memorable experiments which cannot be reproduced here on account of the lack of space, that fermentation is correlative with the life of yeasts. Some years later, he demonstrated the impossibility of spontaneous generation