676 PLANT GROWTH AND PLANT COMMUNITIES 



The slow transfer of strontium in the absence of contact may in 

 part be attributed to a diffusion barrier {e.g., Nernst water film) at 

 the membrane-water and root-water boundaries. Boyd's and HelflFe- 

 rich's "film" diffusion ( Boyd et al. 1947; Helfferich, 1956; Krishnamoor- 

 thy and Desai, 1953) will then be a rate-limiting step. Upon mechani- 

 cal contact, the water films of the two membranes, or root and mem- 

 branes, are bridged by the intermingling ion swarms, and direct 

 particle-particle diffusion is initiated. 



Mattson ( 1948 ) is of course correct when he states : "Whether the 

 uptake of the ions by the roots takes place through direct contact of 

 soil and roots or through the soil solution, the ultimate result should 

 be the same where the three phases are in equilibrium." However, 

 plant growth is a rate process far removed from equilibrium, and for 

 a plant to grow rapidly, nutrients must be delivered to the root surface 

 at a corresponding rate. As will be shown later, contact may become a 

 matter of life and death to the plant. 



Criticisms examined 



Though widely accepted ( Bartlett and McLean, 1959; Cerana and 

 Bielsa, 1959; Deuel et al, 1953; Ratner, 1954; Schweigart, 1956; Wil- 

 liams and Coleman, 1950; Wynd, 1951), the contact theory and its 

 manifestations have been criticized on theoretical as well as on experi- 

 mental grounds. In some instances a two-phase effect could not be 

 observed (Lagerwerff, 1958; Vlamis, 1953); in others it could but 

 was not explicitly recognized as such (Bernstein and Pearson, 1956). 

 In a third group the effect was again present but was explained ( Peter- 

 burgski, 1959) as an H2CO3 carrier problem— a viewpoint previously 

 advanced (Overstreet et al., 1942). We shall examine the most crucial 

 cases. 



Schuffelen ( 1954 ) asserted that plants growing in a clay suspen- 

 sion should be compared not with plants in its equilibrated, superna- 

 tant liquid or dialysate but with plants in an artificial salt solution 

 which has the same cation activity as the clay suspension. In his experi- 

 ments with equal cation activities, the clay suspension and the salt 

 solution were equally effective. 



Bartlett and McLean (1959), who reason like Schuffelen but used 

 Marshall's technique of measuring cation activities in clay suspensions, 

 observed a marked "contact effect"— a superiority of the clay sol over 

 the equal-activity salt solution. 



The activity approach presents an interesting and valuable attempt 

 to assess the total ionic environment of the root. While it does not dis- 

 tinguish between two phases and does not recognize soil solution or 

 ultrafiltrate, it explicitly takes for granted that adsorbed, exchangeable 



