4o6 ABSORPTION OF MINERAL SALTS 



plant. That plants can release adsorbed Ca++ ions from soil colloids in 

 this manner has been demonstrated experimental!}' (Jenny and Cowan, 

 1933) and other kinds of ions undoubtedly can be displaced from the clay 

 micelles in the same way. Since roots are more or less continuously grow- 

 ing through the soil they are constantly coming into contact with additional 

 micelles from which cations can be displaced and absorbed. In many soils, 

 in fact, it is only by continuous growth of the roots through the soil that 

 a continued absorption of mineral salts can take place. The rate of root 

 growth is therefore often an important factor both in the absorption of 

 water (Chap. XVII) and the absorption of mineral salts. As a result 

 of ionic exchanges of this type neutral soils tend to become more acid with 

 continuous cropping unless calcium is supplied from time to time. 



The application of fertilizers to soils will also usually induce cation ex- 

 changes. If an ammonium fertilizer, for example, be added to a soil, some 

 of the NH4+ ions will participate in ionic exchanges with cations already 

 adsorbed on the soil particles. This will result in a lower concentration of 

 NH4+ ions in the soil solution, but also in an increase in the proportion of 

 certain other ions in the soil solution. Similar cation exchanges almost in- 

 variably result whenever other exchangeable cations are introduced into a 

 soil by the addition of fertilizers or in any other way. 



The Penetration of Electrolytes into Plant Cells. — The results of 

 most studies on the permeability of the cytoplasmic membranes indicate that 

 electrolytes, being strongly polar compounds, enter plant cells relatively 

 slowly as compared with non-polar compounds (Chap. X). While it is 

 apparently true that the cytoplasmic membranes are relatively less perme- 

 able to electrolytes than to non-polar compounds, entrance of electrolytes 

 may, under favorable metabolic conditions within the cells, occur very 

 rapidly. For example, Hoagland and Broyer (1936) record that barley 

 roots may absorb significant quantities of certain ions within two hours. 

 The apparent discrepancy between such results and the results of most de- 

 terminations of the permeability of cytoplasmic membranes to electrolytes is 

 probably due to the circumstance that most permeability studies have been 

 made with cells in which metabolic conditions favorable to the rapid en- 

 trance of electrolytes did not prevail. 



It is not known with certainty whether electrolytes penetrate into cells 

 as molecules or as ions. Osterhout (1936) favors the former view. Al- 

 though electrolytes are largely or entirely dissociated when dissolved in 

 water, considerable evidence indicates that the plasma layers of the cyto- 

 plasm are of a lipoidal constitution and electrolytes can dissociate only very 

 slightly when dissolved in such solvents. Other workers, however, believe 



