It may be asked, what proof have we that amongst the filamentous and cellular growths which spring from the small, dark bodies existing in the particles of dust adhering to the surface of fruits, and which we here see bud and multiply with such marvellous rapidity, the ferment or ferments of vintage do actually exist? A very simple experiment will prove conclusively that this is the case. When in the course of twenty-four or forty-eight hours, by contact with saccharine must, and in presence of excess of air, the revival and development of the cells has taken place on the bottom of the little troughs employed in our observations; if then we fill up the trough with the same must, so that there remains no free air under the cover-glass, within a very short time—an hour, half-an-hour, or often less—we shall see bubbles of gas rise from the bottom, accompanied by an increase in the deposit of cells. This will be the must fermenting after the submersion of the cellular plants. It follows that the cells, or groups of cells, of a dark colour which cover the grapes, or the woody part of the clusters, are actual germs of the cells of yeast; more correctly speaking, that germs of yeast-cells exist amongst these groups, for it would not be consistent with truth to say that the various germinating forms present in the dust on the surface of grapes must all of them give rise to actual corresponding ferments. Thus the flask-shaped spores _c_, _c_, ... in Plate VIII., are reproductive organs of _alternaria tenuis_, which have probably nothing in common with alcoholic ferment or ferments, properly so called, except their outward form. We may repeat, however, and it is a point of great importance to bear in mind, that the cells of yeast originate from some or other of the little, brownish, organized bodies, which the microscope reveals in such numbers amongst the particles of dust existing on the surface of fruits. The impossibility, which we have already demonstrated (Chapter III. § 3), of making grape juice ferment apart from the action of external particles of dust, and the knowledge which we have just acquired, that the particles of dust on the surface of the grapes and woody peduncles, at the moment when the grapes have attained maturity, contain certain reproductive cells which give rise to certain ferments, naturally lead us to the investigation of another point, which concerns the period at which these germs make their appearance on the different parts of the vine plant. The two following experiments tend to prove that the ferment can only appear about the time when the grapes attain maturity, and that it disappears during the winter, not to reappear before the end of the following summer.