The Plant as Related to the Soil 



47 



large surface to the action of the root-hairs. Professor 

 King has figured out the result:* "Suppose we take a 

 marble exactly one inch in diameter. It will just slip 

 inside a cube one inch on a side, and will hold a film 

 of water 3.1416 square inches in area. But reduce the 

 marble to one-tenth of an inch and at least 1,000 of them 

 will be required to fill the 

 cubic inch, and their aggre- 

 gate surface area will be 

 31.416 square inches. If, 

 however, the diameter of 

 these spheres be reduced to 

 one-hundredth of an inch, 

 1,000,000 of them will be 

 required to fill a cubic inch 

 and their total surface area 

 will be 314.16 square inches. 

 Suppose, again, that the soil 

 particles have a diameter of 

 one-thousandth of an inch. 

 It will then require 1,000,- 

 000,000 of them to com- 

 pletely fill the cubic inch and 

 their aggregate surface area 

 must measure 3141.59 square 

 inches." All in one cubic 

 inch of soil. When all the 

 surfaces are moist, it is then 

 perfectly plain why a fine soil will withstand more drought 

 and give more root-feeding surface than a coarse soil. 



76. Root-Hairs Absorb Plant Food. Root-hairs absorb 

 the water that covers the soil particles as thin films. 



*King, The Soil. 



Fig. 30. A soil in g< 



