Zinc, Arsenic , and Boron on the Growth of Plants. 301 
barley, as the presence of even 1/5,000 boric acid will admit of a certain 
amount of growth. With decreasing concentrations barley shows a cessa- 
tion of the toxic influence and the setting in of an indifferent action, but it 
is possible that very small amounts may cause improvement in growth. 
Peas, on the contrary, are -very definitely stimulated, the improvement 
usually setting in immediately the toxic action ceases to be evident. The 
results suggest that boric acid does exert a very real stimulative influence. 
With barley, however, the toxic action is rather strong, and makes itself 
felt far down the range of concentrations, so that with intermediate strengths 
of boric acid the stimulative influence is cancelled by the toxic action, giving 
rise to an apparent indifference. With peas, on the other hand, the toxic 
action is far less marked, and apparently ceases with a higher strength of 
boric acid, so that the stimulative influence is able to become evident at 
a higher concentration, and without the intervention of a range of indifferent 
strengths. With lupins no definite conclusions can be drawn, partly on 
account of cultural difficulties, and partly because of the counter action 
introduced by the antiseptic effect of the higher strengths of boric acid on 
the roots, but the probability exists that definite stimulation is caused by 
the weaker strengths of boric acid. 
Thus, with boron the stimulation may be due to a direct action on the 
plant or to an antiseptic action which keeps the roots healthy and free 
from slime. This result of this antiseptic action is visible with lupins, but 
not with peas, so that it is not definitely determined how far the increase 
observed in growth is a direct effect or an antiseptic effect. 
Summary. 
1. Zinc sulphate in high concentration is very toxic to barley and 
peas, and no evidence of stimulation has yet been obtained with any strength 
of the poison down to a lower limit of 1/200,000,000. 
2. Arsenious acid is more toxic in its action on peas and barley than 
is arsenic acid, peas being particularly susceptible to the former poison. 
This distinction holds good for sodium arsenite and sodium arsenate, though 
in a less degree. Again no stimulation is evident with the smallest quantities 
so far tested. 
3. Boric acid is less poisonous than zinc sulphate or arsenic compounds, 
especially with peas. Barley shows stimulation to the eye with some of 
the weaker strengths of poison, but this is not borne out by the dry weights. 
Peas, on the other hand, are definitely stimulated with relatively high 
concentrations of boric acid. The action of the greater strengths of the 
poison is well marked in the leaves, which tend to become brown, and to 
die in a characteristic manner. 
