144 
The Ohio Naturalist. 
[Vol. X, No. 6, 
note the extent in which the results show clearly that the retarda- 
tion in growth of wheat plants is not caused by physical or chem- 
ical conditions but through the direct activity of the bacterial 
flora. It has long been suspected that a reciprocal relation 
exists between groups of soil bacteria and the plants growing 
upon the soil. Various writers have been able to point out that 
marked differences in the productive power of different soils 
followed the growth of wild plants, and that these differences 
persist for some time. It is generallv concluded therefore, that 
the injury caused to cultivated plants by weeds or previous crops 
might be due to influences on the bacterial life in the soil, and in 
Fig. i. Wheat plants growing in i per cent, peptone bog-water solutions 
inoculated with pure cultures of bog bacteria. Numbers correspond with 
data in Table III. 
a direction unfavorable to succeeding agricultural crops. That 
such relations exist the writer is convinced in view of the evi- 
dence presented above. No doubt, the “exhaustion” of soils 
which is frequently met with, and which cannot always be 
attributed to the removal of plant nutrients, is, in part, an allied 
phenomenon. It cannot remain a matter of indifference to 
physiological ecologists whether a strong, intimate, and con- 
trolling relation exists between soil bacteria and surface flora, 
and how the bacterial organisms affect the character, and the 
association and succession of plants. At best very little is 
known of this phase of the physiographic process, and of the 
reactions and effects of the bacterial products upon plant life. 
It would be idle, also, to expect that the bacteriological data in 
themselves are sufficient for a clear interpretation of toxicity 
