HYDRATURE AND PLANT PRODUCTION 275 



weeks in the refrigerator without clanger of chemical changes due to 

 bacteria or fungi. 



3. Deteniiiiiatioiis of the osmotic values of the sap can be acconiphshed by 

 the 'cryoscopic method'. Thereby the freezing point has to be determined. 

 Because this apparatus is generally known we refer to the comprehensive 

 literature given by Walter (1931b) and Kreeb (1961a). Recently Kreeb 

 (1957)5 see also Slavik (1952) used a cryoscope without air space around the 

 sap container and an automatic stirring mechanism. These changes of the 

 apparatus have the advantage of less variation between various determina- 

 tions. Modern cryoscopes are fitted with a freezing apparatus and 

 thermostat.* 



4. Calculation of the osmotic pressure can be done by the formula: 

 W= i2-o6 A (A = freezing point depression). Tables for direct readings are 

 available (Walter, 1931b; Walter and Thren, 1934). 



OSMOTIC VALUE OF PLANTS AND YIELD 



Initial observations regarding the relation between growth and osmotic 

 values (hydrature) were reported by Walter (1931a). He found that the 

 curves of osmotic values from plants grown at different locations {Solamim 

 elaeagnifoliimi) show a good relation to the height of the shoot. Later 

 Stieghtz (1936) worked with winter wheat and tested the production in 

 two different years: In 1934, which was a relatively dry year, osmotic 

 values were high, up to 25 atm, and the yield was only approximately 50% 

 (grain) of the amount in 1933 (year with normal weather). With straw the 

 difference was even greater. 



Lobov (195 1) reports field experiments, whereby various plots, planted 

 with cabbage, potatoes and tomatoes have been irrigated only after a 

 maximum osmotic value of 8, 10 and 12 atm respectively (Table i) had been 

 exceeded. Generally it was found thereby, that with the rise in osmotic 

 values the yield docs decrease. This is not entirely true for tomatoes, as he 

 reported no significant diminution in the yield between 8 and 10 atm. 



Yet it may be asked whether the result would not be different, if not only 

 fruits, but the total production had been measured. This assumption can be 

 made from the fact that tomatoes show a decrease in the production of 

 fruits under extremely moist conditions. This does not dispute the general 

 relationship between yield and osmotic values, but it should be considered 

 because not all factors which affect yield show an influence on the osmotic 



* The cryoscope of Drucker-Burian-Kreeb : designed and manufactured by W. 

 Schulze, Heppcnheim a.d.B., Schunkengasse Nr. 7, West-Germany. 



