182 THE POPULAR SCIENCE MONTHLY. 



tion 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 com- 

 mingled 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 dis- 

 tended, 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 as- 

 similate 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 froth- 

 ing 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 



