I 



THE PATHWAY OF WATER MOVEMENT 87 



II. MOVEMENT THROUGH THE MESOPHYLL 



This problem was approached by constructing a hypothetical model of 

 the leaf cell. The behaviour of the model under certain experimental 

 conditions was studied and experiments performed on the leaf to see 

 whether it behaved in the way predicted by the model. If so, the model was 

 considered to be a good representation of the leaf. 



I. First Hypothesis 



If water moves from vacuole to vacuole as represented in Fig. lA, loss of 

 water by evaporation from cell a will lead to a reduction in its water 

 content and concomitant fall in its water potential. This depression of 

 water potential* will lead to a diffusion of water from cell b and in turn 

 from cell c and so from the xylem element. In this way a water deficit 

 arises in cell a depending on the rate of transpiration and the sum of the 

 resistances imposed by cells b, c, the xylem elements and those resistances 

 lying below (e.g. root resistance). Thus it is the depression of water potential 

 in cell a which is the operative force in moving the transpiration stream up 

 to that point. Fig. iB represents a model of cell a. The apparatus is filled 

 with water, v represents the vacuole and the capillary tube c the resistance 

 in the transpiration stream lying below cell a. The bulb b is porous and as 

 evaporation takes place from it, water is drawn from the reservoir w 

 through the tube c and since this imposes a resistance to flow a reduction 

 of pressure in v arises and mercury is drawn up the vertical tube. This 

 reduction of pressure represents the depression of water potential in the 

 cell. For a given rate of transpiration a certain reduction of pressure will be 

 manifest as the mercury reaching a steady height //, and the volume of 

 mercury occupying the length h in the vertical tube will represent the 

 water deficit in the cell. 



If the tap t is now closed, i.e. evaporation stopped suddenly, water will 

 continue to move into the cell through c whilst the mercury in the vertical 

 tube falls, and finally h will become zerof when uptake through c will cease 

 and the cell will have reached saturation. During this die-away in uptake 

 the rate of uptake^ at any instant will be proportional to ht at that instant, 



/. = ''f (i) 



where r is a constant proportional to the resistance of the tube c. Hence 



* Depression of water potential = D.P.D. = suction force. 



f Differences in level between water and mercury reservoirs are ignored. 



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