448 Plant Physiology 



one another by means of substances derived from their 

 roots. This view was at first credited, but soon lost sup- 

 port. Rotation of crops is based largely upon the idea of 

 physical advantage, or disease suppression. In very 

 recent years some investigators have proposed that soils are 

 commonly unproductive on account of the presence in them 

 of toxic organic compounds. This view with some persons 

 implies that the injurious substances arise through the ex- 

 cretions of roots. The assumption of any general excre- 

 tion of toxic bodies by roots is at present scarcely justified, 

 although an oxidizing power of roots is now demonstrated. 

 273. Unproductiveness. Interesting and valuable 

 data have been accumulated by Schreiner and his asso- 

 ciates, which throw much light upon the nature of the or- 

 ganic compounds which may be found in the soil, and like- 

 wise upon their toxicity. The decomposition of root-hairs 

 and cast-off portions of roots, of green manures, or of any 

 plant or animal remains in the soil give rise to temporary 

 products which may be injurious. Nevertheless, it is not 

 believed that the quantities of injurious organic bodies 

 set free in a well cultivated soil during the growth of a 

 staple crop, whether due to the decomposition of roots or 

 to direct excretion, are often sufficient to be of agricultural 

 importance. With the large number of bacteria ordinarily 

 present in the soil, and the amount of aeration necessarily 

 given in cultivation, such toxic substances would seem to 

 be of merely temporary concern. In the case of bog soils, 

 or land where there is insufficient drainage and lack of 

 aeration, the toxic factor may be permanently important. 

 It is certain that unproductiveness is not due to a single 

 factor of this type, and at present many lines of work are 





