280 



K. KREEB 



of plants will be possible as long as plants are able to grow and to complete 

 their life cycle. 



As a field method for determination of plant hydrature, Kreeb (1957, 

 1961a) suggested measurement of the refractive index rather than the 

 osmotic value of the sap. This can be simply done by a hand sugar- 

 refractometer, giving direct readings of dissolved material, per cent, in the 

 field. As long as changes in the cell concentration due only to water occur, 

 and not due to salts dissolved ( = active change of hydrature) a proportional 

 relationship exists between the osmotic value and the refractive index. This 

 can be seen from the data given in Fig. 5. In this case we used sap of alfalfa 

 which had been diluted with pure water to different levels of concentration. 

 From each of the prepared solutions we determined the osmotic value and 

 the refractive index. The correlation diagram shows very clearly the 

 relation between the values. 



If we do not find a correlation between refractometer readings and the 

 osmotic value in yearly curves, we may then assume that changes in the 



Fig. 6. Yearly curves of hydrature (osmotic values) compared with refractometer 

 values in turnips (above) and lettuce (below). Plots with optimum water conditions. 

 After Kreeb, 1961. 



