Effect of Boron Compounds 75 



kingdom, and it may be useful commercially as a manurial agent, the 

 " catalytic manure " of Bertrand and Agulhon. 



While the higher concentrations of boric acid proved definitely toxic 

 to both peas and barley in the Rothamsted water cultures, some evidence 

 of stimulation was obtained with the lower strengths. With barley the 

 question of stimulation is still an open one, as below the toxic limit 

 growth seems fairly level in most of the experimental series. The 

 lower limit of toxicity varies from 40 4 parts boric acid per 10,000,000 

 according to circumstances. Below this critical concentration the boric 

 acid has apparently no action, either depressant or stimulant, unless 

 the stimulation should prove to begin at a dilution of 1/50,000,000, but 

 the evidence on this point is not sufficiently well marked or consistent 

 to be conclusive. This failure to detect stimulation was somewhat un- 

 expected, as when judged by the eye the plants treated with the lower 

 concentrations of boric acid seemed better than the controls, and also 

 exhibited a particularly healthy green colouration. 



Peas on the other hand are definitely stimulated with traces of boric 

 acid, concentrations of 1/100,000 and less causing an improvement in 

 growth, while under some experimental conditions even higher amounts 

 of boric acid were beneficial. All the stimulated plants showed the 

 characteristic dark green colour which seems to be associated with 

 the presence of minute traces 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, from 1 part in 100,000 downwards. This gave 

 rise to a certain bushiness of growth, which was less evident as the 

 concentration of the stimulant decreased. The general outcome of the 

 tests seems to be that boric acid needs to be supplied in relatively great 

 strength to be fatal to pea plants, and that the toxic action gives place 

 to a stimulative one .high up in the scale of concentration. As far as 

 experiments have already gone it seems as though the stimulation is not 

 a progressive one, as the effect of 1/100,000 boric acid is as good as 

 that of 1/20,000,000, a flat curve connecting the two. This, however, 

 needs confirmation. 



Yellow lupins also give some evidence of stimulation with con- 

 centrations of about 1/50,000 boric acid, the improvement being far 

 more strongly marked in some sets of experiments than in others. 



