430 
Stiles. — On the Interpretation of the 
If one considers the last series only, it will be observed that reduction 
of the supply of nutrient salts to one-fifth of the maximum supplied was 
accompanied by only a slight diminution in the amount of dry matter pro- 
duced. With further reduction the diminution is very marked, and the 
total growth is fairly proportional to the quantity of nitrate supplied. It 
suggests strongly that the amount of salt supplied in the ± strength was 
just a little below the quantity required to allow the other factors present 
full play. A reduction below this critical quantity means that the salt 
supply is a limiting factor. Dr. Brenchley’s published curves show this 
point very clearly. 
One would not, of course, expect an exact proportionality between 
salt supplied and quantity of growth, having regard to the complexity of 
the system and the constant change, not only of other factors but also 
of the rate of salt supply. For the plants grow larger with time, both their 
absorptive and assimilatory systems increasing in size ; but the quantity of 
salt supplied in the same time remains the same. The supply per unit 
of plant is therefore constantly decreasing. It is quite probable that only 
after the plants have reached a certain size will their demands on the 
salts be sufficient for the rate of salt supply to become a limiting factor. 
Thus Dr. Brenchley says, ‘ The development of the shoots in the plants 
growing in the different concentrations was very similar for some long time, 
/ N Nn 
but gradually a falling off was noticed with the two lowest , — J, and by 
harvest time some indications of this appeared even with — shoots’. 
Again, Dr. Brenchley writes that her results indicate ‘ that with the 
lower strengths the amount of growth was strictly limited by the quantity 
of food supplied, and that it was impossible for the plants to reach full 
development with such a restricted amount ’. This is exactly the point. 
The supply of some nutrient salt is acting as a limiting factor. But there 
is no indication that it is the concentration of the nutrient solution that is 
limiting. The possibility of this as a factor will be dealt with later. 
2. Respiration. There seems to have been a strong tendency with 
workers on water cultures to avoid considering the undoubtedly important 
fact that roots respire. In this process roots no doubt absorb oxygen and 
evolve carbon dioxide, and these processes, one would think, must influence 
the rate of growth of a plant in water culture. 
As regards carbon dioxide, it has long been recognized that the con- 
centration of carbon dioxide in the medium external to plant tissue may 
influence its activity. In the case of water cultures the carbon dioxide 
produced by the respiration of the root will dissolve in the culture solution, 
and unless it reacts with any of the dissolved salts its concentration in the 
solution will gradually increase until it may act as a factor limiting 
