n8 Mineral Nutrition of Plants 



largely independent of the water content of the soil and the transpira- 

 tion stream of the plant. To a certain extent, it will occur at o° C. 

 when metabolic activities are near a standstill. 



It may even be envisaged that the entire plant is permeated by electric 

 double layers and ion swarms, and that ions may possess locomotion in 

 all plant parts by virtue of contact exchange and surface migration (20). 



MORPHOLOGIC ASPECTS OF ROOT-SOIL CONTACT EFFECTS 



The contact theory does not invoke the presence of carbon dioxide. 

 Upon contact, the potassium of the clay and the hydrogen of the root 

 directly exchange positions (Figure 5). While the contact theory dis- 



Root 



CONTACT INTAKE 



CONTACT DEPLETION 



Figure 5. Schematic representation of contact intake 

 and contact depletion of cations. 



penses with carbon dioxide it must, on the other hand, assign to the 

 root, properties of an ionic adsorbent, that is, exchange spots. Moreover, 

 the ions assigned to the exchange spots must oscillate sufficiently far to 

 interact with the oscillating ions of the clay particles. 



It is here that some plant physiologists voice their objections. Wan- 

 ner (47) writes "Dieser Mechanismus . . . ist wenig wahrscheinlich, 

 indem dabei die geringe Kontaktflache zwischen Meristem und 

 Bodenpartikel und die mit Wasser imbibierte Zellulosemembran 

 zwischen den beiden efrektiven Adsorptionsflachen nicht beriicksichtigt 

 werden." 



The first objection stresses the small contact zone between meristem 

 region and soil particles. Wanner bases his argument on the salt absorp- 

 tion studies of Overstreet and Jacobson (34) which, at o° C, indicate 

 preferential, strong cation absorption in the meristem region which 



