Page:Popular Science Monthly Volume 59.djvu/502

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GIANT redwood, the monarch of the California forests, stands with its stem-tip three hundred and fifty feet above the soiL From the surface of the millions of tender delicate leaves near the top of the tree there are exhaled many gallons, perhaps barrels, of water daily. The force required to make good this loss is, of course, equal to that needed to raise the water through the three hundred feet or more of vertical space. It is no wonder that the thoughtful person will pause as he contemplates this exhibition of force. It makes no noise; work is being done, but it is not easy to see how.

Let us begin with the soil, as that is the source of the water supply of plants, and briefly consider its constitution, texture and relations to the problem of water-carrying. In other words, does soil carry water and, if so, in what way is it conveyed? Soil is rock that has been broken into small pieces in one way or other, a refinement of rock, so to speak, whether by frost, moving water or chemical action. For our purpose soils, having many degrees of fineness, may be classified into coarse, medium and fine. Coarse soil may be compared to masses of cannon balls touching each other but with large spaces between them, while the medium soil is similar to peas in piles, and the fine soil is like clover seed. The chief difference is in the amount of surface exposed by the particles which go to make up a definite portion of soil.

The next point for us to consider is the capacity of soil for holding moisture. Thrust the hand into a dish of water and upon removal it will be wet, except any portion that has been coated with oil or similar substance. In short, water will leave its own mass and adhere to the surface of the hand. If the hand held a quantity of clean earth the latter would likewise become wet. The amount of water that the soil will hold depends upon the surface exposure of its particles. As this is an exceedingly important point, permit me, at the risk of dealing largely in dry figures but for explanation and proof, to draw upon some results given by Professor King in his charming book 'The Soil.' With columns of sand ten feet long, the one with the grains averaging in diameter 186/10,000 inch, after percolating for 111 days, contained 3.77 per cent, of water; the 73/10,000-inch grains retained 4.92 per cent.; the 61/10,000-inch grains, 5.76 per cent, and the 45/10,000-inch grains, 7.57 per cent. In other words, the smaller the size of the