X.] EXCRETIONS FROM PLANT ROOTS 291 



always being given off by the roots, it has also been set 

 down to fixed acids excreted by the root hairs, and 

 determinations have been made of the acidity of the 

 sap of the roots with the idea of differentiating between 

 the solvent power of various plants. The roots of 

 germinating seedlings are also found on occasion to 

 redden blue litmus paper, and undoubtedly may excrete 

 substances of an acid character, but the behaviour of 

 seedlings, which are building up their fresh tissue out 

 of the broken-down reserve materials contained in the 

 seed, is essentially different from that of plants leading 

 an independent existence, so that nothing is thereby 

 proved as to the source of the etching in Sachs' 

 experiments. 



Czapck instituted a fresh series of experiments with 

 smooth slabs prepared by floating on to glass plates 

 mixtures of plaster of Paris and various phosphates 

 of calcium, iron, and aluminium ; since the iron and 

 aluminium phosphates were attacked, most of the possible 

 acids were excluded, and the etching action of the plant's 

 roots could only be due to carbon dioxide or acetic acid. 

 The latter was again excluded by a further experiment in 

 which the slab was coloured with Congo red, and as this 

 was not affected the sole remaining solvent body the 

 plant could have excreted was carbon dioxide. Again, 

 it has already been shown that water cultures containing 

 nitrates, where the plant is growing in such solutions as 

 exist under normal soil conditions, tend to become 

 alkaline instead of acid, so that the balance of evidence 

 is against the idea that plant roots excrete any fixed 

 acids exerting a solvent action upon the soil particles. 

 The carbon dioxide, however, probably exerts a con- 

 siderable action, especially in the immediate vicinity of 

 the root from which it is given off, for as it passes 

 through the cell wall it must momentarily form a 



