706 



PLANT GROWTH AND PLANT COMMUNITIES 



20 atmospheres is assumed at a root surface, Figure 9 shows the value 

 of the moisture potential in the surrounding soil necessary to maintain 

 a given flow rate from the soil to the root surface. 



The effect of potential in the soil moisture on transpiration rate 

 also has been analyzed by Gardner for plants having leaves with DPD 

 values of 25, 50, 100, and 200 atmospheres. An initial flux of 0.1 ml. per 

 cm. per day to the root is assumed; this, together with the assigned 

 DPD values in the leaves, also defines the integrated permeability of 

 the flow path through the plant. Figure 10 summarizes the calculated 

 effects, which are in good agreement with the experimental results 

 obtained by Slatyer ( 1957), as shown in Figure 11. 



Flow of water through the plant 



Water entering the root must pass through the epidermis, cortex, 

 endodermis, and sometimes other tissues before entering the pipe-like 

 conducting elements of xylem. The water then passes up the xylem 

 along the root and stem into the leaf. Within the leaf, the water must 

 again pass through a series of living, vacuolated mesophyll cells before 

 reaching the air-filled intercellular spaces into which it moves as water 

 vapor. The water vapor then passes through intercellular spaces into 

 the substomatal cavity and finally out between the guard cells of the 

 stomate. Three possible pathways available for the movement of 



0.1 0.2 0.3 0.4 



FLUX- q -ML/CM./ DAY 



Figure 9. Pressure in the surrounding soil necessary to maintain a given rate 

 of flow to a root having a DPD of 20 bars. (From Gardner.) 



