CLIMATIC FACTORS 



129 



Table 8. — Relation of Suction Tension in Bellis perennis to Atmospheric 



Humidity 

 (After Ursprung) 



Osmotic Concentration of Cell Sap. — The investigation of the 

 osmotic concentration of the cell sap of plants yields more constant 

 values which have fewer daily fluctuations and are therefore more 

 useful in plant sociology. The cryoscopic determination of expressed 

 cell sap, so largely employed in America, has been introduced into 

 Europe by Walter (1929, 1929a, 1931), who has improved the method 

 and made it serviceable to plant sociologists. This determination of the 

 osmotic value of cells under normal conditions permits an insight into 

 the water balance of plants and leads to a better understanding of the 

 water economy of the community as a whole. Scattering measure- 

 ments of this type are likely to be of little value. The osmotic values of 

 all the most important members of a plant association should be ascer- 

 tained at intervals throughout the whole growing period or throughout 

 the entire year, in order to obtain yearly osmotic-concentration curves. 

 These curves are likely to be most instructive. 



The data shown on page 130 are from investigations of Braun- 

 Blanquet and Walter (1931) on the range of osmotic values in two 

 associations of theMediterranean region. 



The maximum osmotic value for these plants will be seen to coincide 

 with the long, dry, rainless summer, continuing to the beginning of 

 September. September 7 was the hottest day of the year. A month 

 later, after autumn rains, the osmotic value of most of the plants 

 reached its minimum and then rose very slowly and rather steadily 

 until the next summer. Viburnum tinus, a species with large leaves, 

 little protected against water loss, gave the highest value of 41.6 

 atmospheres. This is an example of a very irregular annual curve with 

 a strong maximum in the summer. 



