THE PATHWAY OF WATER MOVEMENT 



89 



B 



TIME 



TIME 



Fig. 2. A: Model of transpiring cell on the hypothesis that the water moves round the 

 vacuoles, c", capillary tube separating the vacuole from the transpiration pathway; 

 other lettering as in big. lA. B, C and D: Graphs showing the characteristics of the 

 die-away curve of uptake by the vacuole when the tap / is closed. ( r= transpiration 

 rate.) 



2. Second Hypothesis 



If water passes round the outsides of the cells (through the cell walls) 

 rather than from vacuole to vacuole the hypothetical model must be 

 modified. The imphcation now, is that there is a resistance between the 

 pathway and the vacuoles. Such an additional resistance is shown in Fig. 2 A 

 in which the vacuole v is separated from the main stream by a capillary c" 

 of resistance r". With steady evaporation the mercury will rise as before 

 and attain the same steady state position in relation to/o and r as it will still 

 measure the reduction of pressure h at the point p. Closing the tap will, 

 however, have an entirely different effect. During the period of steady 

 transpiration water is dragged through the capillary tube c alone, whereas 



