PLANT PHYSIOLOGY 557 



tent of the environment, either soil or atmosphere, induces a 

 lower osmotic concentration of the cell sap. Wind brings 

 about an increase in the osmotic concentration of the sap, no 

 doubt on account of increase of transpiration and consequent 

 removal of water from the plant. Temperature of the environ- 

 ment appears to be not without influence on the osmotic pres- 

 sure of the cell sap. The osmotic pressure of the cells examined 

 appeared to have a minimum value in the region between io° 

 and 20° C, increasing with change of temperature in either 

 direction. Such a state of affairs is very difficult to explain 

 satisfactorily, and it does not appear that any effect due simply 

 to temperature has been entirely disentangled from that of 

 other factors. As might be expected, illumination, no doubt 

 on account of the production of carbohydrates, brings about 

 an increase in the osmotic concentration of the cells of green 

 leaves. 



As it is clear then that the factors of the environment 

 influence the osmotic pressure of plant cells, it is to be expected 

 that in plants growing in a varying climate there should be a 

 variation in osmotic pressure corresponding to the changes in 

 climate. This has been shown by Ursprung and Blum (" Uber 

 die periodischen Schwankungen des osmotischen Wertes," Ber. 

 deut. bot. Ges., 34, 105-123, 191 6) to be the case, both a daily 

 and an annual periodicity being clearly discernible. With 

 regard to the former, the osmotic pressure increases from early 

 morning to the afternoon, and then falls off again until early 

 the next morning ; as regards the annual periodicity, minimum 

 values are the rule in the summer and maximum in the winter. 



Some contributions to the question of the substances in the 

 cell sap responsible for the osmotic pressure have been made 

 by Harris, Gortner, and Lawrence (" On the Relationship 

 between Freezing-point Lowering, ^, and Specific Conductiv- 

 ity, K, of Plant Fluids," Science, 52, 494-495, 1920, and other 

 papers cited above). They measured not only the osmotic 

 pressure of the expressed sap, but also the electrical conduc- 

 tivity which should give an approximate measure of the content 

 of the sap in electrolytes, though no more than an approximate 

 measure for a variety of reasons. They found, however, that 

 there was no constant relation between the osmotic pressure and 

 electrical conductivity in different species, so that it appears 

 that the ratio of electrolytic and non-electrolytic osmotically 

 active constituents of the cell sap varies greatly from plant 

 to plant. 



In a further series of papers Ursprung and Blum have 

 devoted attention to what they call the " Saugkraft " of the 

 cell (" Zur Methode der Saugkraftmessung," Ber. deut. bot. 

 Ges., 34, 525-539, 1916 ; " Zur Kenntnis der Saugkraft," Ber. 



