88 P.E. WEATHERLEY 



the rate of uptake/^ plotted against the height of the mercury lit will give a 

 straight line as in Fig. iC. It can be shown that the die-away in uptake is 

 logarithmic and may be represented by the equation : 



ft =/oe-«/'-' (li) 



where/o is the steady rate of flow (evaporation) up to the instant of turning 

 off the tap and r' a constant similar to r. Thus a plot of/r against time will 

 give a logarithmic die-away curve as in Fig. iD and plotting the logarithms 

 of/( against time will give a straight line (Fig. lE). The constants in these 



LEAF 



log.ft = log.fo" logizt/'"' 



TIME 



TIME 



Fig. I. A: Diagrammatic representation of a train of mesophyll cells. B: Model of 

 transpiring cell on the hypothesis that the water moves through the vacuoles, v, vacuole; 

 c, capillary tube; w, reservoir of water; t, tap; /), porous bulb. C, D and E: Graphs 

 showing the characteristics of the die-away curve of uptake by the vacuole when the 

 tap t is closed. 



equations are characteristic for a piece of apparatus of given dimensions 

 and all data, say, starting from different evaporation rates would fall on 

 to the same curves. 



