434 
Stiles . — On the Interpretation of the 
(1) No account is taken of the great variation between individual 
plants growing in water cultures under the same conditions. 
( 2 ) In no case has a constantly renewed culture solution been em- 
ployed. Thus the ratio of the various constituents was probably con- 
stantly changing throughout the experiments, and instead of being a 
constant factor was an unknown and varying one. 
Hence the possibility of a lack of balance acting as a limiting 
factor can be admitted, but it cannot be claimed that such a limitation 
has been proved in any one case. 
In the foregoing pages some of the factors which may limit the rate 
of growth of plants in water cultures have been dealt with, particularly 
those which act through the root system, as it is with these that water 
culture experiments, by their very nature, usually deal. In discussing 
these questions frequent recourse has been had to Dr. Brenchley’s re- 
searches, and these have been exposed to some criticism, not because of 
all water culture experiments they are those which most call for criti- 
cism, but because they are those that require it least. They are, for 
instance, apart from the much fewer ones made by the writer, the only 
ones in which the limits of error and significance of differences are 
indicated, ’ and in which therefore recorded differences have a true value. 
They have also been carried out on a generously conceived plan and 
under ideal conditions. Much information is to be derived from them as 
to limiting factors in water culture experiments. But owing in one instance 
to the confusion of two possible factors the conclusions as regards the 
limiting action of concentration do not hold. 
In endeavouring to explain the difference between her results and 
the writer’s, Dr. Brenchley ascribes the failure of concentration of the 
nutrient solution to influence growth in the writer’s experiment to some 
other limiting factor (e. g. smoke pollution) which was ignored. Of course 
the whole point of the experiments was to show that it was some factor 
other than concentration which limited growth. The factor was probably 
largely temperature or perhaps illumination. It may have been atmo- 
spheric impurity as such, though this is not very likely, and it was cer- 
tainly not any such factor as copper in the distilled water (which was 
‘ conductivity ’ water from a block tin still), £ lack of cleanliness in working, 
growth of algae in culture bottles, admittance of light to the roots’. It 
would indeed be reprehensible negligence for a scientific worker to allow 
such factors as these to be operative. 
It should also be pointed out that in dealing with such a complex 
as growth, it is extremely difficult to examine the action of any particular 
factor upon it. During the twenty-four hours a dozen different factors may 
limit growth at different times. Nevertheless it is possible and no doubt 
