186 Walter Stiles and Ingvar Jorgensen. 
concentration changes as the solution becomes more acid or alkaline, 
and this has been known to be the case for fifty years. 
The difficulty arising from changes in the nutrient solution as 
a result of absorption of solutes, can be obviated by continually 
renewing the culture solution, which is most satisfactorily done by 
passing a continuous stream of the solution through the culture 
vessel. Occasional renewal is the next best thing to do. It is 
significant that in all cases where the solution has been renewed at 
intervals increased growth of cultures has resulted, the increase 
being greater the more frequent the renewal (2, 7, 10) and greatest 
when the solution is constantly renewed by passing a continuous 
stream of solution through the vessel. The same treatment would 
also remove toxic excreta if such are indeed produced. In regard 
to changes produced in the concentration of dissolved gases in the 
solution owing to respiration, it is clear that decrease in the oxygen 
pressure outside may decrease the respiratory activity of the root, 
while the same effect may be produced by accumulation of carbon 
dioxide outside. Renewal of the solution will also tend to keep up 
the oxygen supply and to keep down the concentration of carbon 
dioxide. 
The oxygen supply of the solution may also he kept up by 
bubbling air through the solution, and at the same time the removal 
of excess of carbon dioxide would be facilitated. But the expectation 
that aerated cultures would grow faster than non-aerated cultures 
under otherwise identical conditions has not always been realised. 
This diversity of result must again be due to the complexity of the 
processes involved. 
Before leaving the question of the nutrient solution we may 
here make a note on the weighing out of salts for such solutions. 
Great care is often taken to use the purest salts obtainable, which 
are then weighed out with the greatest exactness. Now all our 
experience goes to show that differences of a few per cent in the 
total concentration and in the relative proportions of the different 
nutrients produce no difference in growth which is measurable by 
the water culture method. To obtain such accuracy in the initial 
constitution is simply an unnecessary waste of time when the low 
degree of accuracy obtainable by the water culture method is 
considered, and when it is remembered that the constitution of the 
solution is constantly changing, and that the rate of change will 
depend, among other things, upon the absorptive activity of the root 
system, which is unlikely to be the same in all the cultures in one 
experiment. 
