49 



strycliniue, digitaline, etc., upon animals. Xumerous investigators 

 have obtained similar effects with plants b}' supplj'ing them with very 

 small quantities of various substances which can not be regarded as 

 sources of plant food, such as the extremely toxic salts of some of the 

 heavy metals. In practically all such cases, however, it is very prob- 

 able that considerable hydrolysis had taken place and that the stimu- 

 lation might well be attributed to the hydroxyl ions thus introduced 

 into the solution. 



Raulin experimented extensively with the fungus Aspergillus as to 

 the effect of various metallic salts in stimulating or hindering its 

 growth, his being among the first considerable work in this line.^ 



The well-known observations of Frank and Kriiger ' indicate that 

 copper in small quantities (furnished b}' spraying with Bordeaux mix- 

 ture) stimulates the growth of the potato, acting favorably upon almost 

 every organ and function, although this metal is well known to be 



^ Ann. Sci. Nat., ser. 5, 11, 243 (1869). — The sulphates of zinc and of iron were 



found to produce marked stimulating effect, the former increasing the dry weight 

 of the fungus two to three or even seven times, the latter about twice. In order 

 to show that the acid radicle was not responsible for the results, a corresponding 

 amount i0.06 gram of salt per 1,000 grams of culture solution) of ammonium sul- 

 phate was tried, but no stimulation was obtained. To demonstrate still more com- 

 pletely that basic radicles are here chiefly concerned other salts (nitrates of iron 

 and of zinc, zinc acetate, iron citrate) were tried and yielded stimulative effects 

 similar to those of sulphates. In cases where both iron and zinc were added to 

 the same culture solution i e. g.. zinc nitrate plus ferric citrate, or ferric sulphate 

 plus zinc acetate, or zinc acetate plus ferric citrate) the stimulating effect was 

 decidedly more marked than when only one base was used. When sulphates of 

 both zinc and iron were present the effect was nearly twice as great as in the 

 absence of the former, and was exactly twice as great as in the absence of the lat- 

 ter. The diminution of the stimulating effect was almost as great if instead of 

 merely withdrawing one or the other base an equal portion of the second base was 

 substituted for the first; in other words, when two parts of zinc (or of iron) were 

 substituted for one part each of zinc and of iron. The stimulating effect of the 

 different salts of zinc expresses itself in a crop from two to four and six-tenths 

 times, that of iron in a crop one and four-tenths to two and seven-tenths times as 

 great as in the pure culture solution. 



Manganese was found to produce effects similar to those of iron and of zinc, but 

 "less constant, less appreciable." Silica (as silicates of potassium and of sodium) 

 when added to the culture solution increased the dry weight of Aspergillus in the 

 ratio of 1.2 or 1.4 to 1. 



Raulin wrongly concluded that zinc and silica are indispensable to this fungus, 

 but justly emphasizes •• this influence of infinitely small quantities of substances 

 upon vegetation *' (1. c, p. 253). 



J. Koenig [Landw. Jahrb., 12, 837 (1883)1 ^^^ Griffiths [Journ. Chem. Soc, 

 1884, p. 71, and 1885, p. 461 obtained evidence of a stimulating effect of iron 

 sulphate upon the growth of plants by watering soils used in culture experiments 

 with a solution of this salt. On the other hand Kellner [Landw. Versuchsst, 

 32, 365 (1886)], following the same method of experiment, obtained only negative 

 results. 



^Ber.d.deutsch.bot. Gesellsch., 12, 8 (1894). 



8287— Xo. 71—02 4 



