THE PROTOPLASMATIC MEMBRANE 



and its constituents. It is possible to show this 

 by letting roots grow along polished marble plates. 

 After some weeks the marble surface clearly 

 demonstrates the dissolving effect of growing 

 roots and root-hairs. Delicate traces are every- 

 where etched in the marble surface, where roots 

 have come into close contact with the plate. I 

 was able to show, in 1894, that carbonic acid is 

 certainly to a great extent responsible for this 

 phenomenon. I made plates of plaster of Paris 

 mixed with different substances, the solubility of 

 which in water saturated with carbonic acid, 

 had been well considered. I discovered that 

 only such compounds are dissolved by the plant 

 roots and their excretion, as were distinctly soluble 

 in carbonic acid. These were phosphate of 

 calcium and strontium, but not aluminium 

 phosphate, which is dissoluble by carbonic acid. 

 Nevertheless, there are other effects of acids in 

 plant roots which cannot possibly be due to 

 carbonic acid, and which have not been ex- 

 plained until lately. Now it is believed to be 

 highly probable that merely the adsorption effect 

 takes part in these phenomena, and no excretion 

 of acids by the roots is to be assumed. If the 

 cations are adsorbed and anions of acids remain 

 reactions of acids must result as well as in real ex- 

 cretion of acids. Now we can understand why 

 acids could not be discovered in the excretion 

 drops of the root-hairs, and why they react quite 



