Rate of Ah sorption of Various Salts by Plant Tissue. 499 



dissociation will result from this in all the solutions, which reduces any slight 

 error arising from changes in the degree of dissociation resulting from 

 dilution. Moreover, the greatest increase in the degree of dissociation is to 

 be expected in the solutions from which most salt is absorbed. Hence, with 

 greater absorption, the decrease in conductivity tends to give a value for the 

 absorption less than the true value, from which it follows that changes in the 

 degree of ionisation consequent on absorption of salt and the resulting 

 dilution of the solution will not affect the comparison of the rates of absorp- 

 tion of different salts, except to make the differences appear slightly less than 

 they actually are. 



A more serious complication arises from exosmosis. We have formerly 

 given reasons why, in the case of carrot immersed in solutions of nutrient 

 and harmless substances, this source of error is not of importance, whereas 

 with potato it makes the drawing of conclusions much more difficult. Hence, 

 our conclusions are based primarily on the results obtained with carrot ; the 

 data from experiments on potato are subsidiary to and confirmatory of those 

 from experiments on carrot. Among the results with the latter the toxic 

 action of zinc shows clearly the influence of exosmosis in obscuring the 

 absorption. 



A third difficulty arises if the two ions of a salt are not absorbed equally. 

 Nathansohn (5, 6), Meurer (4), Euhland (10), and Pantanelli(7) have shown 

 by direct chemical analysis that the ions of a salt are not necessarily 

 absorbed with equal rapidity. In the case of inequality of ionic absorption 

 the excess of ion absorbed must be replaced by another ion of the same sign, 

 which may be brought about either by exosmosis from the tissue or by 

 hydrogen or hydroxyl ions. If the absorption of the two ions differs greatly, 

 and if the replacing ion differs markedly in mobility from that of the ion 

 absorbed in excess, the fall in conductivity will no longer give a measure of 

 the absorption. 



Among the experiments recorded in this paper, those with aluminium are 

 probably to be explained on these grounds. It will be recalled, that with 

 aluminium sulphate, although the discs remain turgid and show no sign of 

 injury, yet the conductivity of the solution rises, thus simulating the state of 

 affairs in a toxic solution. Now, Kothert (8) and especially Meurer (4) have 

 shown by direct analysis that the aluminium ion is absorbed to a very great 

 extent from solutions of aluminium sulphate. The results of Pantanelli (7) 

 with a variety of plants are equally striking. We have collected Pantanelli's 

 results with regard to absorption of aluminium salts in the following Table, 

 which show how widely distributed throughout the plant kingdom is the rapid 

 absorption of the aluminium ion in comparison with the sulphate ion : — 



