WATER SATURATION DEFICIT 



109 



authors. Bartos et al. (i960) demonstrated that the intensity of photo- 

 synthesis (which certainly is a sensitive indicator of the physiological state 

 of the tissue) is maintained on the same level for several hours (6 and more) 

 when leaf discs are exposed in a moist polyurethane plate. It thus appears 

 probable that even water uptake leading to WSD saturation will be the 

 same in the disc as in the whole leaf at least for a certain period of time. 

 The saturation curve of young leaf discs has a roughly exponential shape 

 (Fig. 2) which is in agreement with the Uterature (Oppenheimer, 1954; 

 Fritz, 1956). In young leaves, the clear linear phase described by Yemm and 

 WiUis (1954) and amenable to extrapolation could not be found here. It 

 does not appear probable that the extension growth in the disc would 

 proceed at a completely identical rate for the whole duration of exposure, 

 i.e. for 6 hours. In spite of this, the extrapolation suggested by Yemm and 

 WiUis (1954) and appUed here might help to come closer to the true values 

 of actual WSD ; Fig. 4 provides some information about the accuracy of 

 this type of estimation in yoimg leaves. Young leaves display a greater 

 dispersion of values than older leaves. It must be borne in mind, however, 

 that the dispersion in both leaf groups is not due only to an error in 



