The Science of Plant Growing 33 



Work of the Roots. In summarizing the above remarks it may be 

 said that roots fix the plant in the soil, commence to absorb watery solu- 

 tions 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 men- 

 tioned. 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 cur- 

 rent 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 con- 

 ditions 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. 



The Food absorbed by Roots. Of the ten elements of plant food 

 that are absolutely essential, all of them, except carbon and a small quan- 

 tity of nitrogen, are absorbed by the roots. They are oxygen (the free 

 oxygen of the air is used only in breathing), hydrogen, nitrogen, sulphur, 

 phosphorus, potash, calcium, magnesium, and iron. They are not absorbed 

 in this simple form, but in various combinations termed salts (such as 

 nitrates), acids, &c. Oxygen and hydrogen are absorbed in the form of 

 water; nitrogen in the form of ammonia and nitrates; sulphur as sulphates; 

 phosphorus as phosphates; potash and lime in combination with sulphur, 

 VOL. I. 3 



