THE PATHWAY OF WATER MOVEMENT 93 



way and are shown in Fig. 4D. It will be seen that whilst the pathway 

 became virtually saturated after 20 minutes, the inner space would have 

 taken several hours. Evidently in detached leaves at least, rapid fluctuations 

 in water content of the pathway would be accompanied by relatively slow 

 changes in the inner space. But from the point of view of the present 

 discussion perhaps more significant is the fact that the rate of uptake repre- 

 sented hy p (Fig. 4B) is only Jg of the transpiration rate T. In other words, 

 in the case of this Pelargonium leaf water of the transpiration stream moved 

 through the pathway about sixty times faster than water could pass into 

 the inner space. A figure of about fifty times is obtained from Fig. 4A which 

 refers to Popidus candicans. 



No great significance is claimed for the precise values of these ratios. 

 They merely serve to show that water can move through the outer spaces 

 much faster than into the inner space, and it should be emphasised here that 

 the values for the latter are really summated values for all the individual 

 cells each taking up water from its surrounding outer space. If on the other 

 hand water were forced to move through a train of inner spaces in series, 

 the total resistance would be correspondingly greater and the rate even 

 less. 



If the pathway is indeed located in the outer space of the cells, say the 

 cell walls, and the offstream, inner space, is enclosed within a cytoplasmic 

 membrane, not only will the rates of water movement into them be very 

 different, but also they might well react differently to a change in tempera- 

 ture. Mass flow through the cell walls should be rather insensitive to 

 temperature, whilst it is recognised that the permeabiHty of cytoplasmic 

 membranes is often much reduced by a lowering of temperature. To test 

 this, transpiration was stopped suddenly as before except that in this case 

 water at about 3°C was used for immersion. The results of such an experi- 

 ment are shown in Fig. 5 A in which the logarithms of rate are plotted 

 against time. It will be seen that the rate fell away logarithmically with no 

 break until after 22 minutes, uptake had completely stopped. Thereafter 

 no uptake was recorded for a period of 60 minutes when the cold water 

 surrounding the leaf was replaced by water at 23 °C. As will be seen uptake 

 was resumed at once and after attaining a maximum the rate declined 

 logarithmically as in the previous experiments. Thus uptake into the 

 pathway seemed to be little affected by temperature whereas uptake into 

 the iimer space was stopped completely by the low temperature. This is 

 certainly consistent with the hypothesis that movement through the path- 

 way is through cellulose cell walls whilst movement into the inner space 

 of the cells is through cytoplasmic membranes. 



