700 



PLANT GROWTH AND PLANT COMMUNITIES 



-40,000 



-30,000 



-20.000 



•10.000 



F G H J K L 



LEAF — *!-* — ATMOSPHERE^*-! 



Figure 4. Schematic curves showing the distribution of potential in a soil-plant system 

 during three flow conditions. (From Philip, 1957.) 



changes in the moisture content of the plant, it follows as a first ap- 

 proximation that the total flux is constant through each section. There- 

 fore the transmission characteristics of the several sections must difiPer 

 to account for the wide differences observed in the potential gradients 

 throughout the system. The resistance to How (i.e., small value oi k) is 

 greatest at the leaf-atmosphere interface and at the soil-root interface. 

 Figure 4, although only qualitatively correct, serves to emphasize that 

 the water economy of plants is controlled in large measure by the 

 conditions for water transport at the points of entry and exit. 



Flow to the root surface 



The roots of most plants require an external supply of free oxygen 

 for normal growth and activity. Consequently, active roots are found 



