on the Growth of Barley and Wheat in Water Cultures . 87 
The above table shows how great are the discrepancies between the 
results of several workers dealing with the same species by the same method 
of water-culture experiments. In the explicit statement by Cameron quoted 
earlier, it is claimed that the effect of the varying concentrations is largely 
independent of the particular mixture of solutes. If this statement be true, 
it matters little what nutrient solution is used, provided plants will make 
good growth in it, and the argument cannot be advanced that the difference 
in composition of the solution explains the discrepancies between the results, 
concentration being the only point at issue, provided that balance or a given 
ratio of nutrient salts be maintained as far as possible. Stiles 1 probably 
came very close to the truth in saying that ‘ possibly in the American 
experiments something other than concentration of salts was acting as 
a limiting factor in all cases 5 , but apparently he failed to see that the 
same remark may have had a very pertinent bearing upon his own results, 
owing possibly to the smoke factor in the Leeds district. Brezeale 
found with wheat that about 300 parts per million of food-salts gave 
maximum growth, and that growth fell off as the concentration increased to 
750 parts per million or decreased to 75 parts per million, whereas at 
Rothamsted wheat shows a steady depreciation of growth as the concentra- 
tion decreases from 600 to 1 50, well within the former range. Stiles maintains 
that barley grows equally well within the range of 1,800 to 180 pts. per. mil., 
and only shows a slight depression with as little as 90 pts. per. mil., whereas 
at Rothamsted a great and marked decrease in growth occurs from 600 to 
150 pts. per. mil. It seems more than probable that some depressing factor 
must have been at work in the Leeds experiments, which tended to equalize 
the growth of the plants by hindering them in some way which prevented 
normal development and reduced growth to a dead level. Such a factor 
might be provided by the presence of minute quantities of a toxic body 
in the distilled water or in the salts used for making up nutrient solu- 
tions. Experience has shown that the presence of the merest traces of 
copper salts in differential experiments will vitiate growth to such an extent 
as to make comparison useless, and unless the water is prepared with 
the utmost care such toxic substances find only too easy an entrance. 
Unfavourable atmospheric conditions, unsuitable temperature, lack of 
cleanliness in working, growth of algae in culture bottles, and the admit- 
tance of light to the roots are a few of the factors which may adversely 
affect growth, and which have to be taken into consideration in estimating 
results if they come into play. It is impossible to generalize from water 
cultures to sand or soil cultures, or from one species to another, but so far 
as the growth of barley and wheat in water cultures is concerned, this 
last experiment at Rothamsted upholds the earlier contention of Hall and 
Underwood that the concentration of the nutrient solution influences very 
1 Stiles, W. : Review, Journ. Ecol., vol. ii, p. 54 (1914). 
