672 



PLANT GROWTH AND PLANT COMMUNITIES 



tion of the contact theory, provided that rival mechanisms can be 

 ruled out. Under ideal conditions two requirements must be met: (1) 

 at the start of the experiment the separate phases must be in equilib- 

 rium with one another; (2) during the experiment the composition of 

 the phases should not change. 



In the presence of plants the second condition cannot be fully met. 

 It is approached by making the phases large enough so that additions 

 and withdrawals of ions will not significantly change the composition 

 of the phases. If the deviations from ideal conditions are marked, the 

 interpretation of the results becomes correspondingly uncertain. 



Two-phase experiments are performed in various ways. In one set 

 of tests Jenny, Overstreet, and Ayers (1939) separated the suspension 

 and solution phases by a cellulose membrane which permitted ready 

 passage of potassium ions but not of clay particles. This barrier re- 

 duced the diffusion rates of cations by only 10 per cent. Batches of 

 excised barley roots which had accumulated radioactive potassium 

 were permitted to interact for twelve hours with the K-clay suspension 

 directly, in the absence of the membrane, and indirectly, with the 

 membrane present. Release of root potassium was determined by 

 counting the gain in radioactivity of the liquid. The following values 

 were obtained: with no membrane between the root and clay, 76.4 ± 

 5.8 counts per minute; with membrane between the root and clay, 6.8 

 ± 1.8 c.p.m. 



Inserting the membrane brought depletion of the potassium of the 

 root nearly to a standstill. Petersburgski (1959) also used membranes 

 to investigate a solid-phase influence on growth. 



A similar type of two-phase experiment ( Lopez-Gonzales and 

 Jenny, 1958) is illustrated in Figure 2. At the outset the large vessel 

 held 700 ml. of water, containing 122 m. eq. of tagged Sr(OH)2. 

 Beads of H-resin (34.26 g. ) were added. Weeks later the bulk of the 

 Sr(OH)2 had been consumed by the resin, and the solution— now also 



Figure 2. Two-phase experi- 

 ment. Roots are shown in a 

 Sr-resin slurry and in equili- 

 brated solution phase. 



J 



