This section is from the book "Commercial Gardening Vol1", by John Weathers (the Editor). Also available from Amazon: Commercial Gardening, A Practical & Scientific Treatise For Market Gardeners.
In summarizing the above remarks it may be said that roots fix the plant in the soil, commence to absorb watery solutions of plant food at a very early stage; they breathe, and, in the case of land plants, must be grown in well-drained soils, while they are modified in certain plants to perform similar functions in water, or air, and have become fleshy and constitute a storehouse of reserve food in the cases mentioned. Some substances of plant food are soluble in pure water; others are rendered soluble by the presence of carbon dioxide, lime, and other ingredients in the soil. The root hairs and the younger slender fibres of the root are able to dissolve other substances. Their cell walls are actually permeated with acid sap, and this dissolves substances with which they come in close contact. If a small slab of polished marble is placed in the bottom of a flower pot in which a Sunflower, Broad Bean, or Scarlet Runner is grown during the season, and examined in autumn, it will be found that the roots have left their exact impression by eating away the polished surface. If the ingredients of plant food absorbed were to remain unchanged inside the root hairs the sap would soon be of the same density as the watery solution outside the membranous wall, and the inward current would cease; but their chemical nature is continually being changed in one or other part of the plant, and the cells abutting on those having the root hairs absorb the food from the latter, and so on in succession, until it is carried into the vascular tissue of the root, and thence into the stem. This absorption goes on continually night and day, so long as the conditions are favourable. The result is that a current of sap is being pushed into the interior of the plant by the activity of the roots, and is known as "root pressure ", some of the effects of which will be discussed in the chapters on the stem and the leaf. Energy is required by the roots in order to perform all this work, and that is obtained by the absorption of oxygen from the air in the process of breathing. For this reason alone, trees and shrubs should not be planted too deeply, nor should soil be heaped over the surface, where such are already established. We have frequent evidence of large trees being killed outright in a few weeks by the deposition of 3 to 5 ft. of muddy soil or clay over their roots, which cannot breathe nor perform any other function for want of air. Badly drained soils have similarly evil effects. When the soil in flower pots is over-watered, or the drainage hole gets stopped up by worms, the roots cannot get sufficient air, and their functions become deranged, or they die.