16 



THE ECOLOGY OF ROOTS 



dryness which means that while there may be an adequate amount 

 of water present the plants cannot absorb it readily. The reason 

 why the plants cannot get water readily from a salty soil is that 

 the concentration of the soil solution is too nearly equal to that of 

 the cell sap to allow the water to diffuse into the cells rapidly. 

 Obviously the extent of the effect of this factor depends upon the 

 concentration of the cell sap, which varies greatly in different kinds 

 of plants. 



Fig. 3. — Roots of Gleditsia triacanthos showing persistent root hairs. 



It was formerly thought that the absorption of nutrient materials 

 from the soil took place almost entirely from the surface layer to 

 a depth of only 6 or 8 inches. It is now known, however, that both 

 water and mineral salts are absorbed from all layers of soil that are 

 occupied by roots. For example, potatoes absorbed from all layers 

 to a depth of 2.5 feet, barley to 3 feet, and corn to 5 feet. In all 

 cases somewhat larger amounts were absorbed from the upper 

 layers but this was probably due only to the presence of larger 

 numbers of roots in those layers. ]\Iany native plants absorb their 

 nutrients almost wholly below the first 2 or 3 feet of soil. This is 

 necessarily so because there is practically no branching in the upper 



