HILTNER AND STORMER'S "INDIRECT" THEORY 133 



(6) Green plants. 



1. Injurious action as indicated by retarded rate 



and percentage of seed germination and by 

 retarded rate of plant growth. 



2. Beneficial action as shown by increased rate and 



percentage of seed germination and increased 

 rate and amount of plant growth. 



3. Modified in form, color, and other "qualitative" 



changes. 



Hypotheses to Account for Observed Phenomena. A number of 

 hypotheses have been formulated to account for the increased plant 

 growth and for the many changes produced in soils by treatment 

 with heat and volatile antiseptics. A number of these theories 

 are considered, but it must be borne in mind that there is a wide 

 disagreement among workers as to the real cause. No single 

 hypothesis yet formulated can be said to fully account for all of the 

 observed phenomena. 



Koch's "Direct Stimulation" Theory. The first theory offered to 

 account for the increased yield obtained from soils treated with an 

 antiseptic was the "direct stimulation" theory advanced by Koch 

 in 1899. He considered carbon bisulphid to have a direct stimulat- 

 ing effect on the plants themselves. He later found ether to have 

 a similar effect. In experiments dealing with the addition of ether 

 to the soil Koch found that the increased yield was pronounced on 

 the first crop, whereas the residual effect was slight, as with carbon 

 bisulphid the beneficial effect increases with the amount of applica- 

 tion. He further found that soils sterilized with heat produced 

 better crops when treated with carbon bisulphid than when not so 

 treated and concludes that the effect of the antiseptic, therefore, 

 cannot be due to its effect on bacteria. 



The theory of Koch has been supported by Fred who fertilized 

 soil with an abundant supply of sodium nitrate and found that in 

 every case in which carbon bisulphid was added the growth and 

 yield of crop were much superior to those in the corresponding pots 

 not treated with that substance. He concludes that as there was no 

 lack of plant-food and other conditions w r ere favorable to plant 

 growth, the effect of the antiseptic must have been directly upon the 

 plant. There is ample evidence to prove that many of these anti- 

 septics in dilute solutions stimulate the plants directly, yet there 

 is no evidence which will substantiate the claim that this is the only 

 or even the principal influence on the plant and soil. 



Hiltner and Stormer's "indirect" theory of antiseptic action is 

 outlined by them as follows: 



"1. By destroying the existing bacterial equilibrium in the soil, 

 the carbon bisulphid opens the way for an entirely new bacterial 

 development. This is achieved through the unequal retardation 



