Page:Popular Science Monthly Volume 59.djvu/503

Rh grains and the pores the greater the amount of water retained. With soils of still smaller particles the water-holding power would be correspondingly greater.

If the soils are placed in long upright glass tubes and water is added at the bottom, it will rise through the soil to the top. This phenomenon of capillarity is best illustrated with tubes having bores of varying diameter. The tables given us on this subject are to the effect that, when an inch tube is plunged into a vessel of water, the height of the water column in the tube above the general level is.054 inch; for a 1/10-inch bore.545 inch, 1/100-inch bore 5.456 inches and for a 1/1,000-inch bore, 54.56 inches. While the actual surface pull of the smallest tube is much less than that of the largest, it is through a vastly greater distance and by a multiplication of the number of such minute tubes in a given space that the greater lifting is brought about.

The soil itself, consisting of minute particles, admits of the capillary action; for the pores, although not straight, extend in irregular lines and permit the surface tension that is evident in fine tubes. This lifting power of minute passageways is abundantly illustrated in the everyday operations of crop-growing, and the skilful tiller makes abundant use of it or checks it as best suits his purpose. If a dark soil contains an abundance of light alkaline salt, it is possible that it may have a white crust form upon the surface during a drought to be carried back upon the falling of a substantial rain, and this rise and fall may be repeated indefinitely as happens on some of our alkaline lands, where the precipitation is light and vegetation scant.

It has been shown that the soil, on account of its porosity, is able to lift water through considerable distances, simply through the greater pull of a solid for the liquid than the liquid has for its own particles. The hand is wet by the water; a towel hung high with barely one corner dipping into the basin may become wet throughout, and, by evaporation, the dish may be pumped dry.

Into this complex physical porous mixture, to the component particles of which a liquid adheres with such force as to be present when even the air is dry, the plants establish themselves by means of their tiny rootlets and the much more minute root hairs which, insinuating themselves between the microscopic pebbles, become misshapen and contorted beyond all recognition of the simple vegetable cells out of which they have grown. The movements of the water in the soil, whether to the right or left, up or down, are governed, as has been shown, by the law of surface attraction. When we come to the plant cell, the whole physical basis is changed, and, among other things, we are brought face to face with membranes of extreme thinness and delicacy, and, more than all, with the living protoplasmic film.