102 JIRICATSKY 



incurred on application of Stocker's method to not quite mature leaves. 

 The uptake of water by a severed young leaf is not produced only by a 

 momentary incomplete water saturation of the tissues but to a considerable 

 extent also by their extension growth. The saturation to constant weight 

 required by Stocker's method which is reportedly achieved in young 

 leaves within several days actually marks the beginning of leaf dying. 

 From this point of view it is possible to explain even some surprising data 

 found in the Hterature concerning the dynamics of wilting of young and old 

 leaves (e.g. Magyar, 1930; Arvidsson, 195 1) or the annual pattern of water 

 deficit (e.g. Ackley, 1954). 



The possibility of an error in estimating the water deficit in growing 

 tissues was mentioned by Welten (1933) and by Miiller-StoU (1935) who, 

 however, did not consider its value as significant and did not pay any 

 attention to it. More recently the possibility was taken up by Weatherley 

 (1950) and Yemm and Willis (1954). The last-named authors, therefore, 

 when estimating the water deficit of detached whole leaves, determine the 

 shape of the saturation curve and by means of graphical extrapolation 

 subtract the amount of water required for growth. Rutter and Sands (1958) 

 also call attention to the very slow uptake of water by the dwarf shoots of 

 Pinus sylvestris after 10 hours of saturation ; they consider it as a consequence 

 of osmotic changes within the tissue which contained abundant starch. It 

 is very hkely that this 'osmotic uptake' actually plays a role during satura- 

 tion of detached leaves or leaf discs. 



The importance of the WSD values as a rehable indicator of water balance 

 was clearly demonstrated both by the author of the term and by a number of 

 his followers. The weakest point of the investigation of WSD is now 

 apparently the method of its estimation which, strangely enough, has 

 remained practically unchanged since the time of the first of Stocker's 

 papers. It was only Weatherley who (1947, 1950, 195 1) used, for estimating 

 the WSD, discs cut out from experimental leaves and floating on water. A 

 similar method was used by Wormer (1956). Recently a method was 

 suggested for estimating the WSD in discs saturated in a moist plate made 

 of polyurethane foam (Catsky, i960). By using the discs it was possible to 

 cut down the time required for attainment of full turgescence of the tissue 

 and thus to decrease the effect of extension growth on the WSD value 

 obtained. Even here, however, the possibility of a certain even if small 

 error due to this growth persists. 



More complete data on the dynamics of wilting of plants in situ are very 

 rare in the hterature. Important contributions to this problem are cited by 

 Stocker (1929a), Magyar (1930), Arvidsson (1951), Slavik (1955), Halevy 



