274 K. KREEB 



ture inside a cell. It is in equilibrium with the hydrature of the protoplasm 

 (its imbibition) where hfe processes such as growth, respiration and 

 photosynthesis are located. Because it depends on the factors mentioned 

 and upon how much a plant produces, we are practically compelled to 

 understand by the hydrature of plants generally that hydrature inside cells, 

 which can be expressed as the osmotic value of the sap. 



Recently Kreeb (1960b, 1961b, see also Kreeb and Onal, 1961) measured 

 the suction tension using the gravimetric method (Slatyer, 1958) which has 

 been improved and checked with solutions of known osmotic values. 

 Thereby it was foimd in laboratory experiments that in the case of evergreen 

 Mediterranean plants the suction tension may rise more than 60 atm above 

 the osmotic value of the sap in still-hving plants. That means a 'negative 

 turgor pressure' of the cells. Ecologically it does not play a very important 

 role (see Kreeb, 1961b). However in some cases it does exist under natural 

 conditions. In comiection with this we have to ask if the hydrature of living 

 plants can be affected by a negative turgor pressure. It is difficult to under- 

 stand, how this could come about, as it represents only a cohesion tension, 

 which probably does not change the imbibition state of the cytoplasm as 

 in the case of changes in the osmotic value of the sap. For that reason the 

 osmotic value has to be considered as the main factor determining the 

 hydrature of plants. 



METHODS 



I. Sampling. Because of daily changes in the osmotic value of plants, 

 samphng should always be done at precisely the same time of the day. We 

 prefer 12-2 a.m., in order to get extreme values. Insertion, age and general 

 condition of leaves should be considered, because it is very important to 

 have a uniform material which is easily comparable. The proper approach 

 appears to be to collect an average sample at a certain place, using at least 

 20 leaves, leaflets or leaf parts, each having the size of about 10 cm^. This 

 reduces the error of parallel measurements normally to less than i atm. 

 2. Extraction of cell sap. After samples have been collected in glass con- 

 tainers, sealed with rubber corks, they have to be kept in aluminium boxes. 

 One should ensure that the cover of the aluminium box presses the cork 

 well onto the glass container, and therefore prevents water losses by 

 evaporation. After destroying the semipermeability of the cytoplasm by 

 boihng the samples (i 5-20 min in the water bath), sap can be obtained using 

 a hydrauhc press. The press-set, after Walter, was described in 1928. By 

 adding a piece of thymol to the solutions they can be stored for several 



