WATER SATURATION DEFICIT 107 



polation (for the formula, see Methods). It was found by this experiment 

 (Figs. 3 and 4) that the extrapolation procedure is apphcable even to young 

 leaves as the error caused by extension growth is decreased to a very low 

 value. 



4. Origin and Development of the WSD in Plants In Situ during 



Decreasing Soil Moisture 



Experimental plants (a total of 18 plants of fodder cabbage and 36 plants 

 of rape) were divided into groups of six. These groups were then taken as 

 single samples. Before the experiment the plants were weU watered and 

 left overnight in a water-saturated atmosphere. Then three discs were cut 

 out from each leaf of the genetic spiral and their WSD determined jointly 

 in corresponding leaves (i.e. 18 discs from the first leaves, 18 discs from the 

 second leaves, etc.) . The plants were then placed in a greenhouse at 1 8-23 °C 

 and at 65-75% relative humidity and were not watered. During wilting 

 two more samples were removed for estimating the WSD. Figures 5 and 6 

 show the course of wilting of leaves of different age in average values. 



DISCUSSION 



The long-known observations and morphological descriptions of the course 

 of wilting of plants fmd an experimental counterpart in the results of the 

 above experiments. Let us consider first the methodological significance of 

 the described experiments. Their results emphasise very clearly the necessity 

 of careful interpretation of the results of Stocker's method as well as of 

 critical interpretation of the general conclusions advanced in older papers 

 about the water deficit. This is not to say, of course, that Stocker's method 

 in its original form would be of no use or that it would yield nothing but 

 erroneous results. The observed effect of extension growth on the WSD 

 estimation cannot be neglected however, and particularly in cases where 

 the water deficit of the whole plant is to be estimated. The original method 

 of Stocker was intended even for estimations of this type and a number of 

 authors have actually appHed it. On account of the fact that young or at least 

 partially growing leaves represent a considerable proportion of the leaf 

 total in most plants, the utmost care should be exercised in such work. To 

 my mind, this drawback of Stocker's method has probably given rise to 

 the data on the high water deficit of young leaves (e.g. Magyar, 1930; 

 Arvidsson, 195 1 and others) and apparently to the data on higher WSD of 

 plants during the spring months as compared with later seasons (e.g. 

 Ackley, 1954). 



