468 BOTANICAL GAZETTE [December 





may occur and does occur only when iron and zinc are not present 

 in too low amounts. It would seem certain, considering the many 

 described cases of increased growth caused by heavy metal salts, that 

 the contamination of the "stimulants" with iron and zinc could not have 

 occurred without some exceptions if contamination of the "stimulants" 

 were the cause. 



It is not impossible that the action of iron and of zinc and the 

 action of the other so-called "chemical stimulants" will be found to be 

 intimately connected with the increased acidity of the nutrient solution. 

 This hypothesis has already been discussed in a former paper. The 

 assumption of a direct action of the "chemical stimulants" must be 

 dropped, however, since these experiments make apparent that their 

 effects are in some manner related to those of iron and zinc, which latter 

 are directly correlated with acid production by the organism. This ^ 



explanation can also be extended to include the alkaloids, since the ex- 

 tremely high concentrations in which they are effective in bringing about 

 increased growth would eliminate from consideration their action as j 



alkaloids, and their chemical properties as free base and as acid salts are 

 well known. The action of the chemically inert "stimulants," such as 

 ether, chloroform, etc., may in the same manner be laid to the disturbance 

 of the respiration processes leading to an accumulation of organic acids. 

 The fact that free oxalic acid, in common with other acids, can bring 

 about this increased growth, etc., renders this supposition not improbable, 

 in spite of the fact that it is commonly stated that the products of catabo- 

 lism of an organism exercise a harmful effect on its growth. 3 Clearly 

 we have here a case in which a product of the metabolism causes an 

 acceleration of the rate of growth of the organism. | 



These studies must be looked upon, therefore, as a confirmation of the 

 conclusions arrived at by Raulin (Etudes chimiques sur la vegetation. 

 Ann. Sci. Nat. Bot. V. n: 93. 1869) and by Javillier (La Presence et 

 la role du zinc chez la plantes. Theses. Paris. 1908) that both iron 

 and zinc are essential for the growth of Aspergillus niger, and the assump- 

 tion must be made that when iron and zinc are present in favorable 

 amounts a partial replacement of either or both these elements can occur 

 by the so-called "chemical stimulants."— R. A. Steinberg, Bureau of 

 Plant Industry, Washington, D.C. 



^Pfeffer, W., Physiology of Plants. I. Oxford. 1897. 



