An Advance in Root Physiology 



129 



processes taking place in the soil usually result in the oxidation 

 of organic matter to a greater or less degree, depending upon 

 the conditions. This process of oxidation probably occurs, in 

 the case of some substances at least, without catalytic agent of 

 any kind, merely by contact with atmospheric oxygen. In 

 many cases, oxidation is brought about through the activity of 

 microorganisms such as soil bacteria, probably by means of 

 enzymes secreted by the living cells. To the present time, 

 practically all workers in this field have attributed such oxidation 

 in the soil to the growth of these microorganisms. It has re- 

 mained for the Washington laboratory to discover a number 

 of very convincing lines of evidence that the roots of growing 

 plants themselves exert an important influence upon oxidation. 

 Furthermore, it is shown in the present bulletin that certain 

 fertilizer salts greatly . increase the oxidizing power of wheat 

 roots when grown in solutions; that this oxidizing power varies 

 in a general way, with the growth of the plants; and thus that 

 this power is relatively low in extracts of poor soils and high in 

 good ones. This effect of the living roots, however it may 

 prove to be brought about, must be considered as a very impor- 

 tant phenomenon in plant physiology as well as in the study of the 

 soil. 



Two sorts of indicators have proved useful in determining 

 the oxidizing power of roots, soluble substances producing upon 

 oxidation insoluble colored compounds (such as alpha naphthy- 

 lamine, benzidine, etc.), and substances, also soluble, but pro- 

 ducing upon oxidation soluble colored bodies, thus giving a 

 colored solution. Examples of the second class are phenol- 

 phthalin and aloin. 



By the use of the first group of indicators it was possible 

 to locate upon the root the regions where the oxidation takes 

 place. The colored precipitate was formed, and remained upon 

 the surface of the root, in a narrow zone just above the root-cap 

 and in a broad and more intense zone occupying the region of 

 active root-hairs. Higher up, the color fades out, showing that 

 oxidation is progressively retarded as the root-hairs die and fall 

 away. The root-cap and the zone of the greatest elongation 

 failed to exhibit oxidation power. Secondary roots show the 

 same phenomena as primary ones. Where the latter are form- 



