colorless, and branched, and in the aggregate produces in vegetable structure, earth, etc., a white cottony mass, and in liquids cloudy flocks. There are certain modifications of this thallus, but nothing widely divergent from the typical form. It is the mycelium which forms the " spawn" used for planting mushroom beds, and which goes to make up the essential element of "mushroom bricks," which serve the same purpose. The second part of which a perfectly developed fungus may be considered as consisting, is the fructification; and this differs very greatly in the various tribes. In the simplest forms, the fructification is nothing more than a modification of the terminal cell of one of the aërial filaments springing from the mycelium.

One of the simplest, as well as the most characteristic, of the single-celled fungi, is the Torula cerevisia, or yeast plant. A drop of common baker's or brewer's yeast, examined under a power magnifying several hundred diameters, shows itself to consist of a fluid medium, in which quantities of minute spherical or oblong cells are suspended. If this drop be placed in a watch crystal and supplied with nutriment in the form of some fermentable fluid, rich in albuminous matter and sugar, these cells show themselves to be endowed with vitality. [Fig. 17, A.]

There are so few cases where plant growth can be watched, and its results chemically tested, that it will be worth while to follow the development of the torula-cell through the process of its growth and multiplication. In order to do this the cell should be placed in some nutritive medium: this in the case of bread is supplied by the flour and water; in beer by the wort; in wine by the juice of the fruit, or the sugar and fruit juice mixed. The growth and development of the plant produces in these different substances, fermentation, which causes the bread dough to rise; the wort and "must" to be changed into beer and wine.

That we may understand the organic and chemical changes which constitute fermentation, let us “plant” a small quantity of yeast in an appropriate nutritive fluid. The same process then takes place as makes our bread dough rise. Pasteur, the great French chemist, who has done more work and, it is probable, better work than any one else upon the causes of fermentation, has prepared a fluid which perfectly meets the needs of the growing Torula, and makes it possible to watch its development under the most favorable conditions.

A drop of yeast may be placed with a spoonful of

Pasteur's solution in a watch crystal and examined under a power of about four hundred diameters; the larger cells then look about one fifth of an inch long. After a time, close observation detects a growth among the smaller cells, which continues till they reach the maximum size, somewhere near one-threethousandth of an inch. After this size is attained, no change is observed for a time; the little plant seems to be gathering up its forces for the act of vegetative reproduction. Further observation detects in a little while a new growth. From some point in the cell wall a minute projection appears, like a tiny bud; this becomes larger and larger, till a new cell, still attached to the parent form, is matured, and finally equals its progenitor in size. In about three or four hours, under favorable conditions, the new cell begins in its turn to bud, and a second generation is developed in just the same way as the first. So long as the fluid medium affords both sugar and nitrogen, this process continues; when either is exhausted, a new process begins. [Fig. 17, A 1.]

The cells of the Torula, when supplied with the free oxygen of the air, grow with great rapidity, absorbing oxygen and yielding up carbonic-acid gas.

This, it will be remembered, is the process of true respiration, common to all vegetable cells. It goes on continuously in the cells of the green plant day and night; but it is so completely masked by the more powerful process of assimilation effected by the chlorophyll under the influence of sunlight, as to escape observation during the day. The cells of fungi, as we have said, possessing no chlorophyll, lack this power, and with them the breathing process may be observed to go on in light and darkness. alike. As a result of their respiration, carbonic acid is continuously exhaled, and oxygen continuously absorbed.

With the Torula and other fungi, the case stands thus: if confined under an air-tight bell-glass, the cells of the fungus would continually breathe in oxygen, and breathe out carbonic acid, till the limited supply was exhausted, and then perish; it is therefore forced to find some other means of making its living. So long as the Torula has plenty of good fresh air, scarcely any traces of fermentation are to be found; it is merely feeding upon the fluid, as green plants feed upon the nutriment they find in the soil.

But when the cell is sub

merged it turns to the fluid medium not only for

its food, but for air,-for oxygen, which is the element in the air needed for the respiration of plants and animals alike. It forthwith decomposes the sugar in the solution (for all nutritive media. have sugar in them) and appropriates the oxygen which is one of the elements of the sugar, liberating alcohol, which is another of its components. All fermented fruits or preserves give evidence of this; they all have an alcoholic flavor. Bread has not this taste, because the process of fermentation is arrested before much alcohol has been produced, and because, if properly baked, the fumes of the alcohol are dissipated by the heat. If allowed to "rise" too long, too much sugar has been abstracted from the flour and the bread is sour. "The act of fermentation, then, is," in the words of Tyndall, "a result of the effort of this little plant to maintain its respiration by the means of combined oxygen when its supply of free oxygen is cut off." As defined by Pasteur, fermentation is life without air. But it is a severely conditioned life, and "just here the knowledge of that thorough investigator (Pasteur) comes to our aid to warn us against errors that have been committed over and over again. It is not all yeast cells that can thus live without air and