582 Brenchley .— The Influence of Copper Sulphate and 
the time of harvesting, after about seven weeks’ growth, as was borne out 
by means of the dry weights (Curve 4). 
Although the plants with 1 : 10,000 M.S. had very brown roots and 
showed the characteristic deposit in the leaves, yet, so far as weight went, 
there was practically nothing to choose between them and the controls, 
in fact, in some cases, the plants grown with the strong concentration 
of manganese were rather better than those without it. 
Parallel results were obtained from a similar series of ten sets of plants 
grown later in the year. 
It is conceivable that the discrepancy in the results obtained for barley 
was due to the difference in the length of the growing period. In the 
initial experiments the plants were taken through the whole cycle of 
growth, finally ripening their seeds, while in the later tests only the earlier 
stages of purely vegetative growth were passed through. The stimulative 
effect of manganese sulphate of 1 : 100,coo strength and downwards makes 
itself felt from the beginning, and can be traced by means of the periodical 
notes made during growth. The stronger concentration of 1 :10,000 appa¬ 
rently has no effect on the earlier stages of growth, but seems to exert 
a very decided depressing influence when the plants pass through the 
reproductive phases of existence. This depressing effect of manganese is 
shown by its retarding action upon the ripening of the grain, even in 
concentrations which are weak enough to cause stimulation. 
Probably the variation in the results of the earlier and later experi¬ 
ments is partly due to the food solutions used. In the early trials the 
nutritive solution was only half the normal strength, and it may be that the 
masking action of the food salts on the manganese sulphate was less than in 
the later experiments. If this was the case, it is evident that a weaker con¬ 
centration of M.S. would cause depression than with the stronger normal 
nutritive solution. Under these circumstances it may have happened that 
while 1 : 10,000 M.S. exercised a depressing influence with the weaker food 
solution, the same effect with the normal amount of salts would only have 
been obtained with a concentration of manganese sulphate outside the 
range of the experiments under consideration. 
Summary. 
1. The action of plant poisons in dilute solutions is masked by the 
presence of nutrient salts, which thus enable plants when grown in such 
solutions as water-cultures to endure a much greater concentration of the 
toxic substance than in the absence of nutrients. 
2. Copper sulphate, which is a definite poison to Barley, does not have 
any stimulative effect in very dilute solutions, even at so low a concentration 
as 1 : 10,000,000 C.S. 
