1898] RECENT PROGRESS IN ROOT-PHYSIOLOGY 391 



The most important of tlie acid salts is the mono-potassium phos- 

 phate, and this is quite capable of permanently reddening litmus- 

 paper. The other pillar which upholds the view of an excretion of 

 free acids by the root is the fact, first indicated by Liebig and Sachs, 

 that when roots are grown on a polished slab of marble they eat this 

 away in their progress, and mark it out in curious corrosion-figures. 

 By using artificial slabs of known composition {e.g., calcium carbonate, 

 calcium phosphate, aluminium phosphate), Czapek has been able to 

 bring forward very strong evidence that carbonic acid is the only 

 acid responsible for the corrosion-figures. Substances not dissolved 

 by carbonic acid are likewise unaffected by the excreta of the root. 



Whilst accepting these results so far as they go, Pfeffer (12) 

 points out that the question can only be regarded as determined for 

 the particular plants investigated, and for the particular conditions 

 under which the observations were carried out. The fact that we 

 are acquainted with certain fungi which excrete organic acids, and 

 that this formation of free acid is largely dependent on outward 

 circumstances, speaks in favour of this view. " We should not be 

 surprised," Pfeffer remarks, " if certain flowering plants were yet 

 discovered which made use of the undissolved ash-constituents by 

 means of an energetic secretion of acids." 



Of the other substances present in the excretions of the root, the 

 most interesting, perhaps, is formic acid, which had previously been 

 detected by Goebel (5). This acid, Czapek believes, does not occur 

 in the free state, but in combination with potassium. The acetic 

 acid which Becquerel believed he had found in root-excreta could in 

 no case be seen, either free or combined, by Czapek, neither could 

 Boussingault's lactic acid. Oxalic acid, although not generally 

 found, could be clearly demonstrated in one instance, \\2., Hyacinthus 

 orientalis. Enzymes were not found with any regularity in the ex- 

 cretions of the root, and in this respect Czapek's results differ from 

 those of Molisch (7). 



The excretion of the root is generally supposed to pass solely 

 through the root-hairs. Czapek points out that this conclusion is 

 based upon insufficient grounds, and that we even have evidence 

 against it. Eoots of hyacinth grown in water are without hairs, 

 and yet excrete oxalates ; the formic acid is given out by the 

 youngest parts of the root below the region of the root-hairs. 



Another interesting contribution to root-physiology is that of 

 George Peirce (8) on the penetration of roots into living tissues. 

 Pfeffer (9) in a previous research had shown that the roots of 

 the different plants he examined exerted a considerable pressure 

 in their growth. Thus, in one case (Vicia Faba), a longitudinal 

 pressure of 7-10 atmospheres was found to be attained by the 

 growing root. These observations suggested the idea that the 



