96 METABOLISM 



may very well play an important part in the dissolution of soil if these cells 

 be sufficiently numerous. As to the amount of root-cap production we have 

 certainly no accurate information, but our knowledge as to the roothairs is 

 more complete. We know that they live only a few days and are replaced by 

 new hairs formed behind the apex. 



The fact that substances escaping from the plant play some part in the 

 dissolution of the soil must be always borne in mind, since it is certain that the 

 plant can extract from the soil much more in the way of nutrient material than 

 we can with the aid of water holding carbon-dioxide in solution. Thus, e. g., 

 LIEBIG (1862, 2, 108) says : 



' A young plant of rye, grown in a fertile soil, is often capable of developing 

 into a tuft of thirty to forty shoots, each with an inflorescence bearing a thou- 

 sand or more grains, and yet it obtains its nutriment from a volume of soil which, 

 after prolonged washing with pure water, or water containing carbon-dioxide in 

 solution, yields up not one hundredth part of the phosphoric acid and nitrogen, 

 and not one fiftieth part of the potassium and silicic acid which the plant has 

 absorbed from the soil. Under such circumstances how is it possible for the 

 plant to obtain all the materials found in it by merely dissolving them in water ? ' 



In order to obtain some idea how much nutrient material is available in the 

 soil, agricultural chemists employ a dilute (i per cent.) solution of citric acid 

 instead of a solution of carbonic acid ; since it has been shown that the plant is 

 able to absorb as much material as it would do if it excreted weak citric acid. 

 What the materials are which act as solvents, and which are given off by the 

 plant, and whether these spring from living cells or from dead, are matters still to 

 be made out. The corrosion figures are formed on natural or artificial plates only 

 when the substances of which these plates are made possess relatively great solu- 

 bility ; moreover, a root, without forming obvious corrosion figures, can dissolve 

 a considerable quantity of material if it be allowed to operate on a sufficiently large 

 surface ; these are undoubtedly the conditions which occur in nature. Furtherwe 

 must always remember that acids may be excreted only in the presence of 

 definite substances, amongst which, perhaps, aluminium phosphate must not be 

 classed. In this relation, CZAPEK has shown that acid-potassium phosphate may 

 be of service in consequence of the decompositions which it excites outside the 

 plant, e. g. when it reacts with neutral salts of the strong acids, and so gives 

 rise to small quantities of mineral acids. 



Attention has been drawn to the rapid death and constant renewal of root- 

 hairs and, in addition to the close union of roothairs and soil particles,this further 

 point is of special importance in relation to the absorption of nutritive salts from 

 the soil. Numberless roothairs develop from day to day on a large plant, which 

 penetrate into new masses of soil, grow round the particles, taking nutritive salts 

 from them, and rendering naturally insoluble particles soluble. Thus an ever- 

 increasing areaof soil becomes available. Since after each absorption of soil-water 

 at any definite spot a movement of water takes place, tending to produce once 

 more an equilibrium, it might be held that this continual acquisition of new soil 

 particles was of little moment in the absorption of water and substances dissolved 

 in it; but migration of solid bodies is out of the question, and hence the intimate 

 union and constant renewal of roothairs is of the greatest importance. Although 

 the roothairs are the ordinary organs of absorption of nutritive materials, there 

 are many plants which normally possess none. Plants which develop roothairs 

 in ordinary soil, as a rule, do not do so as a rule in water-cultures. In these 

 cases the nutrients are absorbed either by means of the general epidermal cells 

 or and this is a very frequent method by the aid of Fungi which live both 

 on and in the root (compare Lecture XIX). In normal roots, also, the young 

 epidermal cells, which have not as yet developed roothairs, are capable of 

 absorbing nutrient substances (KNY, 1898). 



Finally, a third point may be drawn attention to. Many investigators 



