720 PLANT GROWTH AND PLANT COMMUNITIES 



xylem, the ions are carried passively with the transpiration stream to 

 the leaves, with all the cells adjacent to the xylem acting as ion sources 

 or sinks. 



The possibility has not been excluded, however, that the ion car- 

 rier may be at the surface of the epidermis rather than in the endo- 

 dermis, and that all ion movement in the cortex is metabolic and inde- 

 pendent of water movement in that tissue. Either of these two hy- 

 potheses could account for the known facts. Ion uptake by the plant 

 can be either independent of transpiration or proportional to transpira- 

 tion, regardless of which model is selected, because of the complexity 

 of the anatomy of the plant and of the processes involved, because the 

 original approach of the ion to the root must be through solution, and 

 because the transport of ions up the xylem is largely by movement of 

 ions along with water. 



Summary 



The soil-plant system is highly dynamic. Water, gases, ions, and 

 energy flow through the system in response to potential differences and 

 in accordance with the General Transport Law. Steady-state flow is 

 the exception rather than the rule in the soil-plant system, and the geo- 

 metric complexity of the flow path makes necessary experimental de- 

 terminations of the relevant transmission coefficients. The media 

 through which flow occurs at times serve as either a source or a sink 

 for the material being transported. For water flow, both the amount 

 retained by the porous media and the transmission characteristics are 

 highly dependent on the moisture potential and exhibit hysteresis, 

 which greatly complicates the mathematical analysis of flow in such 

 media. To date no analysis has included the effects of root elongation 

 into new areas of absorption. 



Despite the rather formidable difficulties, considerable progress 

 has been made in the mathematical analysis of flow in the soil-plant 

 system. To apply the General Transport Law, it is necessary to obtain 

 experimental data on the concentration as a function of potential, posi- 

 tion, and time, and of the transmission coefficient as a function of the 

 same three independent variables. Limited data of this kind are avail- 

 able for a few soils, but practically none is available for plants or for 

 an entire soil-plant system. Future progress in the study of the highly 

 dynamic system is dependent upon the development of more adequate 

 techniques of measurements and of computational techniques for solv- 

 ing the complex mathematical expressions needed to more adequately 

 describe transient flow in anisotropic systems which also exhibit hys- 

 teresis effects. 



