the sugar of commerce, or cane sugar, is made up, like starch, of carbon and water; but the proportions differ. Instead of six to five, in saccharose it is twelve to eleven. This sugar is found in the maple and beet; whenever found it is intended as a store for the future use of the plant at the time when a great and sudden demand is made for the purposes of reproduction. Glucose is the sugar met with in the grape and other fruits; it contains a little more water than saccharose and is more soluble. It is necessary that stored-up starch and saccharose should be altered into glucose before they are used by the plant. This alteration is always prepared for by the laying up of nitrogenous matter in close approximation with the stored material. When the food is wanted, the nitrogenous matter acts as indirect ferment, and causes the starch or saccharose, whichever it may be, to take up an additional quantity of water and become glucose. Thus the starch stored up in the barleycorn is altered into glucose when heat and moisture bring the nitrogenous matter called diastuse (which has been laid up under the cuticle) in contact with it, This process takes place in seeds when they germinate, and is taken advantage of by the maltster. For the same reason the tuber of the potato becomes sweet and transparent from the alteration of its sugar into glucose when growth begins. Again, when sugar cane and beet blossom a large supply of nutriment is suddenly wanted; the stored-up saccharose is then digested; that is, altered into glucose, and is carried away in the sap to the reproductive organs, to be there reconverted into starch, and stored up again in the seed. Parsnips and some other sweet roots that do not blossom the first year, lay up glucose itself, which is held in reserve till the next summer, then seed is formed and the root loses its sweetness and collapses.
If yeast be placed in water containing air or oxygen, the oxygen gradually disappears and is replaced by carbonic acid; a process exactly similar to the respiration of fishes, continuing day and night, but proportionately more active. The yeast would die when the oxygen was absorbed, but if glucose be then added, the fungus will abstract from it the oxygen required, and set free carbonic acid and alcohol. Pasteur, who has given great attention to the life-history of ferments, has concluded, after many experiments, that a continued supply of oxygen and the combustion it causes are necessary sources of energy for the development of vitality in ferment plants. As soon as the cells of yeast have exhausted the glucose in contact with them they have a tendency to come to the surface and take on their aerial growth, which is simply the formation of spores. Under favourable circumstances some of the cells at the surface may be observed under the microscope to form an additional internal membrane, which, becoming septose, divides the protoplasm into three or four parts; each of these parts becomes spherical, opaque, and is ultimately detached as a spore. The nutrition of yeast in one particular resembles that of the higher orders of plants, for if is supplied with a soluble nitrogenous ferment which enables it to alter saccharose. This nitrogenous matter may be separated by washing the cells in water, every time they are washed some of it is dissolved out, it is always acid, and if neutralised becomes again acid, directly that it
