682 



PLANT GROWTH AND PLANT COMMUNITIES 



log Fe = 0.0480i + 0.634 ( r = 0.960 ) 



or 



Fe:=4.31 • 



,0.1101 



Surface density of contact sites. Since the plant roots could not 

 have grown unless they pushed the sand grains apart, the contact be- 

 tween the root surface and particles must have been intimate. It is 

 appropriate to express the root-soil boundary as areas. 



The minimal surface area of alfalfa roots was obtained by cutting 

 individual roots into uniform segments, one to ten centimeters in 

 length, measuring the diameter, and computing the surface as tt Id. 

 The mean area of fresh roots of 12 entire root systems was 1.56 ±: 0.042 

 square centimeters for one milligram of dry root, regardless of the 

 size, age, or weight of the plants, as determined in separate trials. 



Figure 9 shows that the iron content in the total plant is directly 

 proportional to the minimal root surface area. 



Experimentally, the mean number of Fe-sand grains per square 

 centimeter of cross-section of the sand medium was found to be 1.98i 

 — 0.20 (r = 0.969). Within experimental error, this mean number 

 ( designated as p ) is twice the Fe-sand percentage concentration. 



Since six transplants had an average dry weight of roots of 3.6 

 milligrams, their fresh minimal surface area was 5.62 cm^. Immersed 

 in sand, this root area was contacted by: 



43 Fe-sand grains in Treatment II (4 per cent Fe-sand) 

 110 Fe-sand grains in Treatment III ( 10 per cent Fe-sand ) 

 222 Fe-sand grains in Treatment IV ( 20 per cent Fe-sand ) 



50- 



O.IIOi 



Fe = 4.3le 

 (r = 0.960) 



Fe = sand, per cent, i 



Figure 8. The uptake of iron by alfalfa plants, as related to the number of 

 iron-oxide-coated sand grains (Fe-sand) in the growth medium. 



