286 Brenckley . — On the Action of Certain Compounds of 
the plants to live for the full time of the experiment, though the develop- 
ment is weak. In only one experiment were such plants found to 
be dying. 
The dry weights of the plants continue to increase until the zinc 
sulphate reaches a dilution of 1/2,500,000 or 1/5,000,000. It is possible 
that the variety of barley may play a part in determining the actual 
critical point, as with ‘Pedigree Plumage’, grown in 1910, the concentra- 
tion was 1/5,000,000, while with ‘English Archer’ in 1911, 1/2,500,000 was 
the strength concerned (Curves 1 and 2). On the other hand, the difference 
may be a seasonal variation, the experiments being made at about the same 
time of year. Below this strength the behaviour of the plants is somewhat 
erratic. In some cases a slight depression remains evident, in others no 
gm. 
Curve 3. Showing the mean values of the dry weights of ten series of barley plants (‘ Archer ’) 
grown in the presence of anhydrous zinc sulphate and nutrient salts. (April 14-May 25, 1913.) 
influence, either toxic or stimulant, can be observed, and again, slight 
increase in dry weight may be obtained with 1/10,000,000 or 1/20,000,000 
zinc sulphate. Such diversity seems to indicate that barley is indifferent to 
the action of zinc sulphate when it is presented in quantities below the 
critical concentration of 1 in 2| millions. 
A further experiment was made to determine if stimulation set 
in at still lower concentrations. A series ranging from 1/5.000,000 to 
1/200,000,000 was tested, and the dry weights of the plants yielded an 
approximately flat curve, the variations being such as would come within 
the range of experimental error (Curve 3). Hence it may be concluded 
that zinc sulphate does not act as a stimulant to barley when grown in water 
cultures, when present in any dilution down to 1/200,000,000. 
Peas. The action of the greater strengths of zinc sulphate on peas is 
parallel to that on barley; 1/5,000 zinc sulphate kills the plants almost 
immediately, without permitting any growth, while 1/10,000 zinc sulphate 
permits a little shoot growth at first, but death ultimately results before the 
dose of the experiment. Even these stunted little plants make great efforts to 
