1913] J- E,. Greaves 13 



arsenic. In fact one, Penicilliiim hrevicaiile, was so adapted to 

 grow in it and evolve diethylarsine that he proposed this as a means 

 of detecting arsenic. Furthermore, one could easily pick out the 

 mixtures containing the greater quantity of arsenic in this work, 

 from the large quantities of mould upon their surfaces. 



The very great stimulating effect of the zinc arsenite in the nitri- 

 fication series was probably due as much to the stimulating influ- 

 ence of the zinc as to that of the arsenic. Lathan^® found that 

 small quantities of zinc stimulated algae. The same results have 

 been obtained by Silberberg^^ working with higher plants. Ehren- 

 berg's^^ work is of special interest in this connection, as he found 

 that zinc stimulated plant growth in soils ; but when the soil was ster- 

 ilized, the zinc became toxic. This would indicate that the Stimula- 

 tion which has been noted by many investigators is probably only 

 an indirect infiuence; and when the soil organisms were killed by 

 heat, the toxic influence on the plant became perceptible. 



Lead arsenate stimulates very greatly the nitrification in soil. 

 Part of this Stimulation is most likely due to the arsenic while some 

 is due to the lead. Stoklasa^*^ has shown that lead, when present in 

 small quantities, stimulates the growth of higher plants. It is pos- 

 sible that the amorphous lead arsenate improves the texture of the 

 soil and in so doing increases nitrification in it. Little if any effect 

 could be attributed to the purely catalytic influence of lead, as Russell 

 and Smith^*^ found this to be very small. Again there is the possi- 

 bility that some of the Compounds inhibit the activity of injurious 

 species. But these possibilities, and the question as to how much 

 the results would vary in other soils, are problems which can be 

 solved only by further work. Stoklasa^^ ascribed the observed 

 Stimulation of growing sugar beets, when arsenic and lead were 

 applied to the soil, to the catalytic action of these Clements on the 

 chloroph>dl apparatus of the plants. These results indicate that it 

 was due to their influence on the biological transformation of the 

 nitrogen in the soil. 



16 Lathan : Bul. Torrey Bot. Club, 1909, xxxvi, p. 285. 



17 Silberberg : Ibid., 1909, xxxvi, p. 480. 



18 Ehrenberg: Landw. Vers. Stat., 1910, Ixxii, p. 15. 



19 Stoklasa : Compt. rend., 1913, elvi, p. 153. 



20 Russell and Smith : Jour. Agr. Sei., 1906, i, p. 444. 



21 Stoklasa : Expt. Sta. Rec, 1912, xxvi, p. 225. 



