Respiration as a factor in the ionic equilibria between plant tissues and external solutions

Abstract

Many investigations have been made of the salt exchange between slices of plant storage organs and solutions in which they have been placed. A simple form of experimentation is to place the pieces of tissue in water devoid of solutes, and to investigate the drift with time of the concentration of the external solution. The concentration of such a solution was used by Stiles and Kidd (1919) as a base line from which to calculate the absorption of salts by the tissue when placed in a salt solution. For example, the conductance of distilled water rose by 190 units in 52 hours when carrot tissue was placed in it, whereas that of 0·0002 N KCI increased only 137 units, whence it was deduced that the carrot tissue had absorbed KCI to the extent of 53 units of conductance. The justification for this procedure is doubtful, since a solution with which the tissue was in equilibrium would show no change of conductivity, and yet by this method of reasoning would be said to have lost salt to the extent of 190 units of conductance. More recently stiles (1927) has made detailed studies of the problem of the exosmosis of substances from slices of various storage tissues, including carrot root, into distilled water. Well-washed discs of tissue were placed in water and shaken continuously at 20° C. The drift of the conductance of the outside solution was followed. It rose for about 24 hours and then fell slowly for about 10 days, after which a steady value was maintained for some time until the conductance rose as the tissue died. The concentration of the outside solution was also determined by the weight of the residue after evaporating the solution to dryness. The concentration so determined followed the same course as the conductance. Stiles suggests that the initial liberation of electrolytes, which is greater the shorter the preliminary washing, takes place from the cut surface-cells and from the veils underneath, which soon die. These electrolytes are absorbed by the remaining living cells at a rate which is eventually greater than that at which they are liberated, with a consequent tail of the external conductivity. The tissue, small in bulk relative to the solution, would have to absorb the greater part of the electrolytes liberated. There is no direct evidence in favour of such an explanation. As far as we know, tissue, such as Stiles used, never decreases the conductivity of such dilute solutions, except in experiments of the type which we are trying to explain.