54 Effect of Arsenic Compounds 



phosphoric acid, arsenic acid causes a strong production of organic sub- 

 stances up to the flowering time. The following figures were obtained 

 with maize : 



002 gm. As 2 3 with P 2 O 5 2 -84 gm. dry wt. 



005 gm. 2-37 



01 gm. As 2 6 67-32 



40 64-13 



03 As 2 5 without P 2 6 39-98 



07 42-13 



normal solution 12'93 



with 65-84 



Comparative experiments with the two arsenical oxides showed that 

 varying times were required to kill different plants. Young seedlings 

 were brought into solutions containing 1/10,000 mol. wt. arsenious acid 

 (='019 gm. As 2 O 3 per litre) and the plants died in a very short time. 



Hordeum distichum 46 hours 

 Polygonum Fagopyrum 84 

 Persecaria 90 



With ten times the strength of arsenic acid (1/1000 mol. wt. = '23 gm. 

 per litre) the plants took much longer to kill. 



Hordeum distichum 24-5 days 

 Polygonum Fagopyrum 40 

 Persecaria 42 



Various experiments have been carried on at Rothamsted with peas 

 and barley. With arsenious acid on barley a depressing influence is 

 manifest even at a concentration of 1/10,000,000, while no growth at all 

 is possible with 1/10,000 and upwards. Apparently the toxic action on the 

 root ceases at a higher strength than on the shoot, as with 1/1,000,000 

 and less the dry weight of the root remains practically constant. At this 

 same strength the shoots look better than the controls, but this is not 

 apparent in the dry weights (Figs. 9 and 10). With peas the depression is 

 again evident to 1/10,000,000, but the plants are more sensitive to the 

 higher concentrations, as rib growth can take place in the presence of 

 1/250,000 arsenious acid (Fig. 11). A striking difference is observed 

 with arsenic acid on barley, as apparently this does not act as a toxic 

 even with such comparatively great concentrations as 1/100,000, 

 though possibly the shoot is slightly depressed by this strength 

 (Fig. 12). 



