Effect of Boron Compounds 69 



at 100 mg. boric acid per litre life seems impossible for the plant. The 

 roots seem to be more adversely affected by toxic doses than do the 

 shoots. In control plants Agulhon determined the stem/root ratio as 6, 

 with a little boron as 1, while the ratio rose to 13 as the dose of the 

 poison increased to 50 — 100 mg. boron per litre. 



The Rothamsted experiments show that boric acid is definitely 

 poisonous to barley down to a strength of 1/250,000 (Fig. 15), the de- 

 pressing effect frequently being evident at much smaller concentrations, 

 while peas can withstand far more of the poison, the limit of toxicity 

 being about 1/25—1/50 thousand (Fig. 16). With the greater strengths 

 of poison the lower leaves of both barley and peas are badly damaged. 

 In barley the leaves turn yellow with big brown spots, giving the leaves 



gm. 

 I>4 





Shoof 



Rooh 



300 100 20 



* 2. I -4 .2 .| .04 -OZ O 



l>|:|,000.000 



Fig. 15. Curve showing the mean value of the dry weights of ten series of barley plants 

 grown in the presence of boric acid and nutrient salts. (May 1st — June 20th, 1911.) 



a curious, mottled appearance, while with peas the poisoning seems to 

 begin at the tip and edge of the leaves, spreading inwards, without, 

 however, showing the large spots as in barley. So far as chemical tests 

 go at present, it is very probable that boron is deposited in the leaves 

 in the same way as manganese, and that this is the cause of the de- 

 generation. As with manganese, the lower leaves are attacked first, and 

 the trouble spreads upwards, one leaf after another being involved. 

 These observations fit in very well with those of Hotter, and the 

 hypothesis of direct boron poisoning gains support from the fact that 

 in dilutions which produce stimulation of the shoot the leaves show 

 hardly any sign of dying off, even after prolonged growth in the 

 solutions. With barley the effects of boron can be seen in the leaves 



