K.— BOTANY 227 



giving rise to Donnan equilibrium, it is inevitable that the movement of 

 ions demanded by these conditions must result. 



The possibility that respiration has a direct effect in bringing about 

 the absorption of ions has been pointed out by several workers, notably 

 by Briggs and S. C. Brooks. The production of carbon dioxide in the 

 cell leads to the appearance of carbonic acid and hence of its ions hydrogen 

 and bicarbonate, H and HCO3. The interchange of ions required by the 

 Donnan equilibrium will lead to the diffusion out of hydrogen ions which 

 are replaced by kations from the external medium, while the bicarbonate 

 ions will be exchanged for anions from the external medium. As the 

 tissue continually respires the production of hydrogen ions continues to 

 replace those which diffuse into the external solution, and so the absorption 

 of ions continues as part of an interchange between tissue and external 

 medium. 



An interesting theory of salt absorption which hypothesises some sort 

 of combination of the absorbed ions with constituents of the protoplasm 

 followed by passage of the ions into the vacuole through exchange with 

 hydrogen and bicarbonate ions, has recently been proposed by S. C. 

 Brooks. According to this theory, the substances in the protoplasm 

 responsible for the initial absorption are the proteins. In the protoplasm 

 are proteins of various kinds, which are differently ionised, some with the 

 protein group carrying a positive charge, others with the protein ion 

 carrying a negative charge and thus constituting a proteinate ion. When 

 a salt such as potassium chloride is absorbed the potassium ion unites 

 with a H-proteinate and the chloride ion with a protein-OH. The 

 potassium proteinate and protein chloride thus produced unite with the 

 basic and acidic groups of adjacent molecules and so move through the 

 protoplasm until they reach molecules adjacent to the vacuole. Here 

 exchange with H and HCO3 ions produced as a result of respiration is 

 supposed to take place. 



Against the view of a direct effect of respiration on salt intake by ionic 

 exchange it has been urged by Hoagland and Steward that accumulation 

 of ions is negligible or slight when tissue is deprived of oxygen, although 

 there may be a considerable anaerobic production of carbon dioxide. 

 But as regards this objection it must be noted that under conditions of 

 anaerobiosis the rate of carbon dioxide production usually falls rapidly 

 with time, so that it is doubtful whether a considerable production of carbon 

 dioxide anaerobically generally continues for any length of time. The 

 question is obviously one requiring further experimental investigation. 



That the absorption of salts by tissues is related to a supply of oxygen, 

 and probably in some way to respiration, there can, however, be no doubt. 

 As long ago as 191 3 Hall, Brenchley and Underwood showed that barley 

 and other plants in aqueous culture solutions grew more rapidly in 

 aerated solutions than in non-aerated ones, an observation which was 

 confirmed by Jorgensen and myself in 19 17 in regard to barley and balsam 

 and by Knight in 1924 with wallflower, Chenopodium album and Elodea. 

 The conclusion could be drawn that in these experiments the augmenta- 

 tion of the oxygen supply to the roots brings about an increase in the 

 rate of absorption of the nutrients necessary for metabolism and growth, 



