82 PHYSIOLOGY. 



173. Absorption of soluble substances. Since these substances are dis- 

 solved in the water of the soil, it is not necessary for us to dwell on the 

 process of absorption. This in general is dwelt upon in Chapter 3. It 

 should be noted, however, that food substances in solution, during absorp- 

 tion, diffuse through the protoplasmic membrane independently of each 

 other and also independently of the rate of movement of the water from 

 the soil into the root hairs and cells of the root. 



When the cells have absorbed a certain amount of a given substance, no 

 more is absorbed until the concentration of the cell-sap in that particular 

 substance is reduced. This, however, does not interfere with the absorp- 

 tion of water, or of other substances in solution by the same cells. Plants 

 have therefore a certain selective power in the absorption of food substances. 



174. Action of root hairs on insoluble substances. Acidity of 

 root hairs. If we take a seedling which has been grown in a 

 germinator, or in the folds of cloths or paper, so that the roots are 

 free from the soil, and touch the moist root hairs to blue litmus 

 paper, the paper becomes red in color where the root hairs have 

 come in contact. This is the reaction for the presence of an acid 

 salt, and indicates that the root hairs excrete certain acid sub- 

 stances. This acid property of the root hairs serves a very im- 

 portant function in the preparation of certain of the elements of 

 plant food in the soil. Certain of the chemical compounds of 

 potash, phosphoric acid, etc., become deposited on the soil par- 

 ticles, and are not soluble in water. The acid of the root hairs 

 dissolves some of these compounds where the particles of soil are 

 in close contact with them, and the solutions can then be taken up 

 by the roots. Carbonic acid and other acids are also formed in 

 the soil, and aid in bringing these substances into solution. 



175. This corrosive action of the roots can be shown by the well-known 

 experiment of growing a plant on a marble plate which is covered by soil 

 In lieu* of the marble plate, the peas may be planted .in clam or oyster 

 shells, which are then buried in the soil of the pot, so that the roots of the 

 seedlings will come in contact with the smooth surface of the shell. After 

 a few weeks, if the soil be washed from the marble where the roots have 

 been in close contact, there will be an outline of this part of: the root sys- 

 tem. Several different acid substances are excreted from the roots of 

 plants which have been found to redden blue litmus paper by contact 

 Experiments by Czapek show, however, that the carbonic acid excreted by 

 the roots has the power of directly bringing about these corrosion phenom- 



