y8 Brenchley —Effect of Concentration of the Nutrient Solution 
water cultures are compared with those for thousands of plants grown 
at Rothamsted during the last nine years, it is seen that they are remarkably 
low, so low as to suggest that some factor was in action at Leeds that was 
quite ignored or overlooked in the estimation of results. Plants are very 
sensitive to external influences other than those of food and water-supply, 
and the amount of light, variations of temperature, and the atmospheric 
conditions prevailing during the growing period all have definite action on 
the rate and quality of growth. Crowther and Ruston 1 have shown that 
the smoke pollution of the air at Leeds is so great that plant life is most 
seriously affected, considerable depression in growth being caused at the 
University. This factor must have operated upon the water cultures, and 
may, to some extent at least, have vitiated the results obtained. Whereas 
at Leeds in 1914 the mean dry weight of barley-plants grown from 
April 28 to June 6 was only 0-628 grm., at Rothamsted one series grown 
simultaneously from April 27 to June 9 averaged 2-516 grm. dry weight, 
another series averaging 2-252 grm., and this was in spite of the advantage 
gained by the Leeds plants in the frequent renewal of food solution, while 
the Rothamsted plants remained in the initial solution all through the 
experiment. 
It has been stated that £ plants growing in water cultures under exactly 
the same conditions are very variable ’, 2 and this is used as an argument for 
discounting the value of water cultures as a method of experiment. 3 As 
a matter of fact, the individual variation of plants within a single series is far 
less than with similar plants growing under natural conditions in the open 
field. It is only necessary to examine carefully a small area of barley in the 
field, plant by plant, and to compare with a number of water cultures growing 
at the same time, in order to be convinced of the truth of this fact. Dactylis 
glomerata is on the whole a bad subject for water-culture experiments, but 
even in this case the range of individual variation under such conditions is 
most obviously less than between plants growing on the experimental 
plots. Mean variation from series to series is fairly great, because the period 
of the year has a very great influence upon the rate of growth, and plants 
grown in January and February may possibly not reach one-quarter the 
development (as shown by dry weight) of similar plants grown in April and 
May for the same length of time. Experiments have shown that the differ- 
ence of even a week in putting plants in water cultures has a distinct effect 
upon the total dry matter that can be produced within a given time. Every 
experimental method has its disadvantages and its weaknesses, and while 
water-culture methods are far from perfect, and indeed make no claim to be 
so, yet they do afford those conditions that are the most under the control 
1 Crowther and Ruston : Town Smoke and Plant Growth. Journ. Ag. Sci., vol. vi, Pt. iv, 
pp. 387-94. 2 Stiles, W. : loc. cit., p. 89. 
3 Stiles and Jorgensen : Studies in Permeability, I. Ann. Bot., vol. xxix, p. 349 (1915). 
