70 Effect of Boron Compounds 



in concentrations as low as 1/2,500,000, and it may be significant that 

 this is the point at which the depressant action of boric acid entirely 

 ceases in many cases. 



Tests with white lupins gave no conclusive results, as for some 

 reason it proved very difficult to get satisfactory plants in water cul- 

 tures. When they are grown under such conditions the roots always 

 tend to get more or less diseased and covered with slime, probably 

 fungal in nature. In the presence of much boric acid the roots 

 remain in a much healthier condition, which suggests that the acid 

 has in this case a strong antiseptic action, and protects the roots. With 

 high concentrations the lower leaves of the plant are badly affected, 

 just as with peas and barley, turning brown and withering at an early 

 date. Various experiments have been made with yellow lupins, but 

 these again are very difficult to grow well in water cultures, as they are 

 apt to drop their leaves for no apparent reason. Generally speaking, 

 the evidence goes to prove that boric acid is toxic down to a concen- 

 tration of about 500 parts in 25 million. It is difficult to get a true 

 control with which to make comparisons as the plants without boric 

 acid are encumbered with the slime on their roots, which naturally 

 interferes with normal growth, while the plants in the presence of 

 boric acid have the unfair advantage due to the probable antiseptic 

 action of the boron. The effect of the boron poisoning is again evident 

 in the dying off of the lower leaves, which become flaccid and drooping 

 and finally drop off. The lupins grown with boron are very active in 

 the putting forth of lateral roots, so much so that the cortex of the 

 roots is split along the line of emergence of the laterals, which are 

 very numerous and crowded. 



(b) Toxic action of boron compounds in sand cultures, 



Agulhon (1910 a) moistened 2 kgm. pure sand with 500 c.c. nutri- 

 tive solution for each pot, and boron was added at the rate of 0, O'l, 

 1, 10, and 50 mg. boric acid per litre of nutritive solution. Twenty 

 wheat seeds were sown in each pot, and after twelve days the healthy 

 plants in the first four pots were 6 8 cm. high, but those with the 

 maximum amount of boron showed yellowish leaves only 3 cm. long. 

 After three months' growth the plants were harvested, when those with 

 most boron were found to have died after making about 10 cm. growth. 

 The toxic doses in sand proved to be weaker than those in water cul- 

 tures, probably because evaporation from the surface of the sand caused 

 concentration of the poisonous liquid. 



