74 Effect of Boron Compounds 



of boric acid being added in addition. *1 mg. boric acid per litre of 

 N.S. ('05 mg. per pot) gave an increase of 25 / fresh weight, and 7*5 / 

 dry weight. The stimulating doses seem to be weaker than in the 

 experiments with liquid media, probably because the evaporation from 

 the sand increases the concentration of the boric acid at the surface. 

 It was also noticed that the increase of weight varied in experiments 

 made at different times. With oats the stimulating influence is greater 

 than with wheat, showing that some plants are more sensitive than 

 others to the influence of boron. With radish 1 mg. boric acid per litre 

 exercised a stimulating effect, the enormous average increase of 61 % in 

 fresh weight occurring with this strength, though this only represented 

 an average increase of 9'6 / dry weight. 



(c) Soil cultures. 



Here again the stimulating action was evident with higher concen- 

 trations than in sand cultures, and Agulhon obtained good results 

 with strengths that are toxic in sand. The evaporation from earth is 

 not so rapid as from sand, so that the concentration is not increased, 

 and also some of the boric acid is withdrawn from the solution by 

 interaction with the soil, so that the stimulating concentration rises in 

 the scale. 



In field experiments Agulhon found that peas were more sensitive to 

 the toxic action of boric acid than is maize. A strength of boric acid 

 (= 1 gm. B. per sq. metre) that poisoned peas, gave an increase of 61 / 

 fresh weight and 39 / dry weight with maize ; half the strength 

 proved to be indifferent for peas, the improvement with maize equalling 

 56 % increase fresh and 50 / increase dry. Curiously enough, judging 

 by appearances in the first experiment, an unfavourable influence was 

 at work, though in reality a great stimulation was being caused. Colza 

 gave a good increase with similar strengths, but with turnips 1 gm. B. 

 per sq. metre only favoured the aerial parts, while '5 gm. B. per sq. 

 metre only increased root development. Agulhon concluded that it is 

 as yet impossible to determine with any precision the exact part that 

 boron plays in the plant economy. He suggests that boron is a 

 " particulier " element characteristic of a certain group of individuals or 

 of life under particular conditions. In his summary he argues that each 

 series of individuals adapted to different environments has doubtless 

 need of particular elements, and that perhaps chemical causes and 

 morphological differences are very closely connected. Boron may be 

 of this "particulier element" type in the higher plants of the vegetable 



