Zinc, Arse?iic , and Boron on the Growth of Plants. 295 
From a comparison of curves and figures it is evident that boric acid 
has a decidedly toxic action on barley, if it is supplied in too great amount. 
Below a certain concentration, about 1/250,000 boric acid, the action is 
indifferent, but there are indications that very low strengths, below 
1/10,000,000, may exercise a stimulant action, but the concentrations have 
not yet been carried down low enough to ascertain this definitely. The 
results were somewhat unexpected, as, when judged by the eye, some of 
the indifferent concentrations of boric acid seemed to cause strong stimula- 
tion, the colour of the plants also being particularly healthy. 
200 IGO 20 to 4 2 1-4 -2 -I -04 02 0 
1 = 1 : 1 . 000,000 
Curve 15. Showing the mean values of the dry weights of ten series of barley plants (‘English 
Archer ’) grown in the presence of boric acid and nutrient salts. (May i-June 20, 1911.) 
Peas. In the initial experiments in July and August, 1910, the range 
of concentrations was from 1/5,000 to 1/20,000,000 boric acid. Even with 
the strongest of these concentrations the plants were not killed, but they 
were very poor and weak. All the lower leaves died and the edges of 
nearly all the remaining leaves were brown and shrivelled, while the roots 
were curiously long and thin. As the concentrations decreased the plants 
improved until with 1/100,000 boric acid they seemed normal to the eye, 
the dry weights indicating some stimulation with this strength. All the 
stimulated plants showed the characteristic dark green colour which seems 
to be associated with the presence of boron in the nutritive solution. An 
interesting morphological feature was the strong development of small side 
shoots from the base of the plants in the presence of medium amounts of 
boric acid, 1/100,000 downwards. This gave rise to a certain bushiness of 
growth, which was less evident as the concentration of the stimulant decreased. 
With plants grown later in the same year, in September and October, 
it was found that 1/1,000 boric acid was necessary to kill the plant at once. 
Even with as high a concentration as 1/2,500 the peas made some little 
attempt to push out shoots before succumbing, but the root growth was 
x 2 
