THE SOLVENT ACTION OF ROOTS 53 



solution of the phosphates in question contains insufficient 

 phosphoric acid for the needs of the plant. 



Another class of experiments are often quoted in support 

 of the theory of the excretion of a permanent acid. If the roots 

 of a young seedling be pressed between blue litmus paper, a 

 permanent red coloration ensues, and that this is not due to 

 breakages of the cells and simple extrusion of the sap may be 

 seen from the fact that water in which seedlings have been 

 germinated also possesses an acid reaction. Czapek (loc. cit.) 

 satisfied himself that the acid reaction found in these cases was 

 due to potassium hydrogen phosphate. It has again been stated, 

 though there is some doubt about the correctness of the data, 

 that young seedlings put to grow in a solution of ammonium 

 chloride induce a permanently acid reaction — a fact which, if 

 true, points much more to the preferential selection of the 

 ammonia than to the excretion of any acid from the root. It 

 is, indeed, not always possible to observe the alleged blueing 

 of litmus when the roots of young seedlings are brought in 

 contact with litmus paper, and Czapek admits that the drops 

 of liquid, always to be seen standing on the root-hairs of seed- 

 lings developing in a damp atmosphere, possess no acid reaction. 

 In any case, however, these phenomena exhibited by seedlings 

 have but little bearing on the question at issue ; the life of a 

 seedling goes on by the breaking down of elaborated reserve 

 materials and their transport to the growing points where 

 reconstruction takes place — a wholly dissimilar process to the 

 inflow of simple nutrients and their removal from free solution 

 which marks the action of a normal root. Nor is it credible 

 that any growing plant, once past the seedling stage, will part 

 with so valuable a material as potassium phosphate. 



Another method of attacking the problem has been to sum 

 up the actions of the plant upon the soil by taking out a 

 balance-sheet between the acids and bases contained within 

 the completely developed plant. In a normal soil the plant 

 has to obtain its nutrition from neutral salts, except so far 

 as their solution is rendered acid ;by the presence of carbon 

 dioxide; if, then, the plant excretes any fixed inorganic acid, 

 the plant itself must be left with an excess of base. Now, 

 though the ashes of a plant are alkaline in reaction through 

 the presence of an excess of base, yet the ash no longer 

 contains the nitrogen, all of which entered the plant in the form 



