Page:Popular Science Monthly Volume 30.djvu/196

182 a simple osmotic apparatus. Osmosis there undoubtedly is, but it is exactly similar here to osmosis everywhere, and, while accounting for certain things as capillarity accounts for certain other things, still does not mean growth. Let us see what osmosis can do. If two liquids of unequal density be separated by a membrane pervious to either or both, an interchange between the two fluids occurs until equilibrium of density is established, the greater quantity of the commingled fluids being found at last on that side of the membrame at first occupied by the denser fluid. Suppose now, for illustration, a chain of cells extending from some leaf on the maple-tree down to some rootlet in contact with a drop of water, each cell-content of less density than that above it, and we should have a current setting toward the leaf, and likewise, though less in energy and amount, a current in the opposite direction. Certainly, something of this kind actually happens, not in a single row of cells, but involving all active cells of the tree, so that water from the soil is carried to the leaf, and the products of the latter are diffused throughout the organism. We may even conceive the cells beneath our block of wood to be distended to repletion by the process just described, yet all this is not growth. Given this machinery at the beginning of our experiment, and we can see that the connection of the block would be strained as when wooden wedges, by absorbing water, burst the rock. But the cells once distended, the limit of pressure is reached, and everything would remain in statu quo. And now appears the energy of life's forces. After osmosis and diffusion have done their best, the living matter of the cell is able, notwithstanding the pressure, to enlarge the cells, increase their number, and thicken their walls, and this it is that at length produces the phenomenon we have seen, and brings the spikes, heads and all, through the yielding wood.

But let us look at another example illustrating this same thing. In the manufacture of beer, as every one knows, the alcohol of the beverage is produced by fermentation, a process induced through the activity of brewer's yeast. Now, brewer's yeast, as may be shown by any good microscope, consists essentially of minute single cells, each of which is capable of performing alone all vital functions; i. e., each cell can assimilate food, grow, and reproduce its kind—the two functions last named being here, as elsewhere, dependent on the first. The food of the yeast-cell in this instance is grape-sugar or glucose. From this comes as a sort of by-product of assimilation carbon dioxide in large quantities. The liberation of this gas in the wort produces the frothing which constitutes so noticeable a feature of fermentation. The glucose being the source of supply whence the gas is eliminated, it is plain, all questions of temperature aside, that gas will appear so long as glucose remains in sufficient quantity to nourish the yeast. The amount of glucose found in different grades varies, but certain it is that no beer is entirely free from this yeast-food, so that, when the