666 



PLANT GROWTH AND PLANT COMMUNITIES 



extent selective and not directly proportional to the concentration of 

 the soil solution. Some ions, especially potassium, are taken up prefer- 

 entially; others, such as calcium, are partly excluded. 



Matter that leaves the root may be grouped as follows: (1) Water, 

 which may be lost from roots in dry soil ( Breazeale, 1930; Stone, 1957 ) . 

 (2) Carbon dioxide, produced by root respiration. (3) Nutrient ions, 

 as leakage when the salt concentration in the root is high (Helder, 

 1956 ) , or, when it is low, in the presence of clay ( Jenny and Overstreet, 

 1939). (4) Protons diflFusing out from the organic-acid pool (Over- 

 street ef al., 1942), possibly not related to respiratory COo. (5) A flow 

 of electrons, as postulated by Lundegardh (1958). (6) Organic sub- 

 stances, e.g., organic acids. Information is meager and conflicting, and 

 it is difficult to separate active excretion from mere autolysis— uis:., the 

 release of material from injured and dying cells (Fuss, 1956; Helder, 

 1956; Starkey, 1958). 



The fluxes take place in the liquid phase, but such a statement is 

 too broad to be meaningful. Although growth media consisting of 

 quartz sand and nutrient solution conform to the common idea of liquid 

 phase and solid phase, in fine-textured soils, in which colloidal parti- 

 cles abound, the sharp distinction between solid and liquid becomes 

 blurred, and boundary and surface phenomena become increasingly 

 important. 



Model of root-soil boundary region 



The patterns of influx and outflux are bound to be influenced by 

 the submicroscopic. architecture of the root surface. An attempt is 

 made in Figure 1 to portray the boundary zone as it might be envisaged 



Cell wall 



ti 







I00m>i 



200A 



ljLi=IOOOmM=IO.OOO A 



Figure 1. Model of root-soil 

 boundary region, based on 

 the work of Frey-Wyssling 

 and of Scott et al. The cell 

 wall consists of cellulose mi- 

 crofibrils (w), pectic sub- 

 stances (p), and free space 

 (/), the pore diameter of 

 which is exaggerated. The 

 thin plasma membrane, SO- 

 SO Angstrom in diameter, at 

 the boundary of cytoplasm 

 and cell wall is shown, but 

 sketchily. The black bodies 

 are clay particles (cl); B is a 

 small bacterium; u is a virus 

 particle. 



