It is clear that under these circumstances not only water enters the root-hairs, but many of the substances found in solution in the soil-water enter the plant also.  Among these are the mineral substances which are indispensable for the proper life and growth of plants.  These plant nutrients are so indispensable that if any one of them is absent, it is absolutely impossible for the plant to continue its life functions.  The indispensable plant-foods gathered from the soil by the root-hairs, in addition to water, are:  potassium, calcium, magnesium, iron, nitrogen, and phosphorus,—­all in their proper combinations.  How the plant uses these substances is yet poorly understood, but we are fairly certain that each one has some particular function in the life of the plant.  For instance, nitrogen and phosphorus are probably necessary in the formation of the protein or the flesh-forming portions of the plant, while potash is especially valuable in the formation of starch.

There is a constant movement of the indispensable plant nutrients after they have entered the root-hairs, through the stems and into the leaves.  This constant movement of the plant-foods depends upon the fact that the plant consumes in its growth considerable quantities of these substances, and as the plant juices are diminished in their content of particular plant-foods, more enters from the soil solution.  The necessary plant-foods do not alone enter the plant but whatever may be in solution in the soil-water enters the plant in variable quantities.  Nevertheless, since the plant uses only a few definite substances and leaves the unnecessary ones in solution, there is soon a cessation of the inward movement of the unimportant constituents of the soil solution.  This process is often spoken of as selective absorption; that is, the plant, because of its vital activity, appears to have the power of selecting from the soil certain substances and rejecting others.

Movement of water through plant

The soil-water, holding in solution a great variety of plant nutrients, passes from the root-hairs into the adjoining cells and gradually moves from cell to cell throughout the whole plant.  In many plants this stream of water does not simply pass from cell to cell, but moves through tubes that apparently have been formed for the specific purpose of aiding the movement of water through the plant.  The rapidity of this current is often considerable.  Ordinarily, it varies from one foot to six feet per hour, though observations are on record showing that the movement often reaches the rate of eighteen feet per hour.  It is evident, then, that in an actively growing plant it does not take long for the water which is in the soil to find its way to the uppermost parts of the plant.

The work of leaves