Influence of Aeration of Nutrient Solution. 185 
A view which has been held for many years is that the rate of 
growth depends on the hydrogen-ion concentration. Thus we find 
it asserted that it is necessary to keep the culture solution slightly 
acid and that chlorosis results if the solution becomes alkaline. In 
recent years there have been so many observations on the importance 
of hydrogen-ion concentration in physiological processes that there 
is every reason why the earlier observation of the importance of 
slight acidity should have been made. 
The concentration of phosphate dissolved in the solution has 
also been held as of importance, but in spite of Benecke’s attempt 
(1) at an investigation of the complex physical chemistry of water 
culture solutions it is not clear whether it is hydrogen-ion 
concentration, or the concentration and solubility of phosphate, iron, 
or other constituents which is of importance in the avoidance of 
chlorosis. 
A possibility which has never been properly examined is that 
of changes with time in the composition of the dilute solution which 
constitutes the nutrient solution. It is possible that gradual 
changes of concentration of the various constituents may take place 
owing to such processes as hydrolysis and double decomposition, 
which will result in changes in hydrogen-ion concentration and 
phosphate concentration among others which may influence the rate 
of growth. 
But this is probably a small matter in comparison with changes 
which take place in the nutrient solution as a result of plants growing 
in it. Causes working to alter the composition of the solution are ab¬ 
sorption of constituents, and absorption of dissolved oxygen and 
excretion of carbon dioxide in respiration. There is also the possibility 
of the production of toxic substances in the solution either as the result 
of active excretion, as some American writers would hold, or as the 
result of decomposition of dead root tissue, 
The absorption of constituents from the solution reduces the 
total concentration of the solution, but as the rate at which the 
constituents are absorbed depends not only on the concentration of 
the constituents outside, but also on the rate at which they are 
removed as such inside the plant, it is clear that the solution 
external to the roots must change in relative composition as well as 
in total concentration. Moreover, it appears that the constituent 
ions of any salt are absorbed independently of each other (6). As 
a result not only do the relative proportions of the constituents of 
the nutrient solutions alter with time, but also the hydrogen-ion 
