90 P.E. WEATHERLEY 



after the tap has been closed and the mercury begins to fall, water must be 

 dragged through both capillary tubes c and c". Thus the rate of uptake will 

 instantaneously fall from that given by eq. i to: 



/ = — (.») 



After the instantaneous fall, / will decline logarithmically according to 

 eq. ii. Thus the curve of rate of uptake (Jt) plotted against time will appear 

 as in Fig. 2B and the logarithmic plot as in Fig. 2C. If the rate of uptake is 

 plotted against the water deficit h (Fig. 2D) instead of a straight hne there 

 will be an instantaneous fall from each transpiration rate (Ti, T^, T3 etc.) 

 on to the general straight line defmed by eq. i though in this case the 

 resistance is (r+r"). It wiU be seen that these two hypothetical models 

 predict that if transpiration could be stopped suddenly, the pattern of 

 continued uptake of water in response to the water deficit in the mesophyll, 

 would be entirely different according to whether the water of the 

 transpiration stream moves round the cells or through them. 



3. Experimental 



Data were obtained using leaves o£ Pelargonium zonale, Popuhs candicans 

 and Rihes sanguineum. In each experiment a detached leaf was fitted into a 

 simple potometer as shown in Fig. 3. Uptake was measured by following 

 the movement of a meniscus in the horizontal tube using a travelling 

 vernier microscope. At first a steady measured rate of transpiration was 

 maintained for one or two hours. Transpiration was then stopped suddenly 

 by immersing the leaf in water or medicinal paraffin by raising a beaker 

 from below and the die-away in uptake followed. Similar results were 

 obtained using either water or medicinal paraffin. In the course of each 

 experiment the total amount of water virtually to saturate the leaf was 

 measured by the potometer and clearly this represented the initial water 

 deficit in the leaf Further, the water deficit at any time was measurable as 

 the amount of water subsequently absorbed to attain full saturation. Typical 

 results are shown in Fig. 4A, B and C. Clearly all these results fit the second 

 hypothesis and not the first. 



The experimental results do however differ from those predicted by the 

 second hypothesis in that the drop in rate on stopping transpiration is not 

 exactly vertical (Fig. 4B). This is undoubtedly due to the pathway itself 

 having a water deficit and taking up water to saturation. This uptake by the 

 pathway appears to die-away logarithmically hke that of the inner space of 

 the cells but much more rapidly, indicating a low resistance between itself 



