494 



ECOLOGY 



The advantage of such migration often is assumed, it being asserted 

 that roots exhaust the food materials in any given portion of the soil. 



However, it is likely that the amount 

 of mineral matter used by plants is so 

 small, and that the supply in the soil 

 commonly is so great, that uncultivated 

 soils rarely if ever are exhausted. Fur- 

 thermore, plants sooner or later return 

 to the soil the mineral matter that 

 they took from it. Probably the 

 " sterility " of many soils is due less 

 to the abstraction of necessary elements 

 by vegetation than to the addition of 

 deleterious substances, which may be 

 definite root excretions, or which may 

 arise through the decomposition of 

 organic material in the soil. 



Very recently it has been shown that 

 the roots of certain plants excrete sub- 

 stances which impede further root ac- 

 tivity. If this phenomenon proves to 

 be general, as now seems likely, the in- 

 vasion of new soil areas by roots may 

 make possible their escape from the 

 substances which they give off or which 

 arise by subsequent decay. Even in 

 the case of cultivated crops, it is prob- 

 able that fertilizers are of less value as 

 sources of plant food than in their action 

 upon soil constituents and in counter- 

 acting the noxious effect of root excreta 

 or of decaying vegetation. Certain root 

 enzyms are oxidizing agents of much 

 importance and assist in the destruc- 

 tion of the deleterious soil compounds ; 

 however, when these compounds are 



FIG. 706. Seedlings of maize 

 (Zea Mays) that have grown in moist 

 air just above water; note that the 

 submersed portions of the roots arc 

 hairless and exhibit irregular growth 

 curvatures; the older leaves (s) en- 

 sheath the delicate younger leaves; 

 maize seedlings illustrate hypogaean 

 germination. 



present in excess, the oxidizing action becomes lessened and the addi- 

 tion of nitrates and of other fertilizer salts is of great value. Farmers 

 have long believed that fields occasionally should lie fallow ; the ad van- 



