98 P. E. WEATHERLEY 



the door open' to the water flux in response to the purely physical force of 

 a pressure difference. 



IV. CONCLUSIONS 



An analysis of the die-away curve of continued water uptake by a detached 

 leaf when transpiration is suddenly stopped, can be understood in terms of 

 a model which suggests that the water in the leaf consists of two phases : 

 the pathway of transpiration itself and a second phase lying off-stream and 

 separated from it by a barrier of relatively high resistance. Thus at the 

 instant of stopping transpiration, water was found to pass into the second 

 phase at only about J^ of the rate of the transpirational uptake. 



In view of the cell consisting broadly of regions of high permeabihty 

 (e.g. cell walls) on the outside separated by barriers of low permeabihty in 

 the cytoplasm from imier regions (e.g. vacuoles), it seems likely that the 

 transpiration pathway is the 'outer space' of the cells and the off-stream 

 phase is the 'inner space'. The experiments which have been described do 

 not tell us precisely whether the outer space of the cells includes the cyto- 

 plasm or is the cell wall alone. The seeming lack of sensitivity of water 

 uptake into the outer space to temperatures a little above freezing point 

 suggests perhaps that the cytoplasm is not included. The cytoplasm might 

 well be in a gel state at these low temperatures when uptake of water would 

 be mainly diffusional and probably too slow for the observed rapid 

 absorption. In this connection it is interesting that from their study of the 

 water relations of leaf discs Carr and Gafr(i96i) have concluded that the 

 cell-wall water is a significant and important fraction of the leaf water 

 content and indeed their evidence strongly suggests that the outer space 

 referred to in this paper should be regarded as the cell wall. We have 

 obtained no information on the relative volumes occupied by the inner 

 and outer spaces ; this is clearly essential for ascertaining their nature and 

 will be the subject of further work. 



The conclusion that the water deficit in a leaf can be partitioned into 

 phases is perhaps a side-issue from the point of view of this discussion. 

 Nevertheless it may have important physiological, ecological and even 

 technical implications (Carr and Gaff, 1961 ; Barrs and Weatherley, 1962). 

 For example the inner space, at least in detached leaves responds to changes 

 of external water potential at only /^ of the rate of the outer space, and 

 this could lead to the inner space being cushioned from external fluctuations 

 as envisaged by Carr and Gaff This aspect clearly deserves further intensive 

 study. 



