428 Stiles. — On the Interpretation of the 
the chief may be considered as assimilation and respiration. Any factor 
which will influence either of these will also influence growth. Assimilation 
in the wide sense may be dependent upon all those environmental factors 
that influence carbon assimilation, chief among which are temperature, 
carbon dioxide, and light. In addition it may be influenced and there- 
fore limited by factors acting in the first place through the root system, 
water-supply, and supply of nutrient substances in the solution surrounding 
the roots. The respiration will certainly be influenced by the concentration 
of oxygen and carbon dioxide in the solution surrounding the roots, and 
the concentration of these gases in the nutrient solution will thus influence 
growth. 
Besides these obvious factors, others are possible. For instance, the 
concentration of the nutrient solution might influence growth by affecting 
the absorption of necessary salts, or by influencing transpiration. It has 
been shown by the writer 1 that under certain conditions the concentration 
of the nutrient solution over a fairly wide range of variation does not act as 
a limiting factor in growth in water culture. Recently Dr. W. Brenchley 2 
has asserted the contrary, her conclusions being based on several series of 
experiments in which the growth of plants was apparently dependent on 
the concentration. It will be shown here that Dr. Brenchley’s results are 
in all probability due to the confusion of two possible factors — the absolute 
amount of salt available, and the concentration of the solution ; and that in 
her experiments the growth was limited, not by the concentration but by 
the insufficiency of the total quantity of salt supplied, the initial concentra- 
tion of the solutions not being maintained throughout her experiments. 
The Factors. 
In discussing the various factors which may influence the rate of 
growth of plants in water culture, only those will be considered which act 
through the root, as it is for the purpose of investigating these that water 
culture experiments are usually carried out. At the same time it must be 
recognized that factors acting through the subaerial part of the plant may 
limit growth, and that generally during the twenty-four hours of any day 
a series of factors probably limits growth at different times. Only a very 
approximate parallelism can then be expected between the rate of growth 
and any particular factor even when this is the chief limiting factor during 
the day. 
1. Supply of ‘ Nutrient ’ Salts . As the plant body consists of 
complex substances which contain various elements only derivable from 
salts present in the solution surrounding the roots, it is clear that if the 
1 Stiles, W. : On the relation between the Concentration of the Nutrient Solution and the Rate 
of Growth of Plants in Water Culture. Ann. of Bot., vol. xxix, 1915, pp. 89-96. 
2 Brenchley, W. E. : The Effect of the Concentration of the Nutrient Solution on the Growth of 
Barley and Wheat in W T ater Cultures. Ann. of Bot., vol. xxx, 1916, pp. 77-90. 
