436 Stiles. — On Water Culture Experiments. 
2. Water culture experiments are usually performed in order to examine 
the influence of certain factors upon growth, by controlling the action of 
these factors acting through the root. 
3. Owing to the neglect of the possible effect of other factors, and 
to the difficulty of controlling most of these factors in water cultures for any 
length of time, it may be difficult to obtain definite evidence of the relation 
between any one factor and the rate of growth. 
4. It should be obvious as an elementary physiological principle, and it 
is amply borne out by the results of published experiments, that the supply 
of nutrient elements in the water culture solution can act as a limiting 
factor. 
5. With this factor has been confused that of the concentration of the 
nutrient solution. No evidence has yet been obtained that concentration of 
the nutrient solution acts as a limiting factor, but it is probable that it may 
so act in exceedingly dilute solutions, and it probably does in very high 
ones. 
6. The possibility of the influence of respiratory activity of the root on 
the culture solution has been neglected hitherto. This may play a part in 
limiting growth. 
7. The water culture method is of very limited application to physio- 
logical problems on account of (1) the difficulty in analysing results due to 
the complex of factors not under control ; (2) the difficulty of controlling in 
some cases even the factor whose action is being investigated ; and (3) the 
excess of labour required to produce results which are only of a low degree 
of accuracy. 
London, 
June , 1916. 
