130 



The Plant World 



ing and are about to push through the cortical tissue, oxidation 

 is active, as is shown by the development of colored spots upon 

 the surface of the primary root. Of several lines of evidence 

 that the oxidation here considered is not due to the growth of 

 microorganisms upon the root surface, perhaps the most con- 

 vincing is the fact that the zone of dead and dying root-hairs 

 (where the growth of microorganisms should- be most active) 

 showed little or no reaction. 



Phenolphthalin. which gives a colorless solution, is oxidized 

 to the well known phenolphthalein by the action of plant roots 

 or other oxidizing catalyte. After the roots have been in con- 

 tact with the solution for ten to twenty hours thev are removed 

 and the solution made distinctly alkaline with sodium hydroxide. 

 Thus the red alkaline color of the oxidized product is developed, 

 and the amount of oxidation is quantitatively determined by 

 the intensity of the color. Aloin solution oxidizes from a pale 

 vellow to a deep wine red. and the color change may likewise 

 be measured colorimetrically. 



Many treatments of poor soil extract which are beneficial to 

 growth also accelerate this extra-radical oxidation. Thus, 

 treating the extract with absorbing solids had this effect, as 

 has also distillation of the extract. Nitrates usuallv accelerate 

 root oxidation. It is suggested that the beneficial effects pro- 

 duced by fertilizers may often depend upon an acceleration of 

 the oxidation process, an entirely new hypothesis in regard to 

 fertilization. 



The presence of toxic organic compounds in the culture 

 medium greatly retards oxidation, but the activity of the roots, 

 espeeiallv if accelerated by the presence of a nitrate, finally 

 diminishes or removes the toxic property. 



From an incomplete study of the nature of the oxidation 

 process, the authors conclude that it is probably mainly depend- 

 ent upon peroxidase excreted from the roots. 



