168 SOME RECENT RESEARCHES IN PLANT PHYSIOLOGY 



In all cases the age of the leaf was found to be of more 

 importance in defining osmotic pressure than was anything 

 else, with the exception of diurnal fluctuations resulting 

 from rapid alterations in carbohydrate content. In a 

 previous chapter it has been shown how large variations 

 in the relative proportions in which the leaf sugars are 

 present may take place without altering the molecular 

 concentration of the cell sap. The osmotic pressure 

 depends, of course, upon the molecular, and not the 

 percentage composition. 



Since all these determinations were made with sap 

 pressed directly, a new series of experiments on Syringa, 

 Hedera, and Ilex, were instituted by Dixon and Atkins 

 (1915, 2). In this the tissues were rendered permeable 

 by treatment with liquid air. The results obtained 

 showed that the former values were too low. They, 

 however, substantiated the occurrence of large fluctuations. 

 The foregoing figures illustrate these researches. 



From Figs. 20, 21, and 2lA, it may be clearly seen how 

 pronounced is the effect of treating the tissues with liquid 

 air, for the curves furnished by the sap of the frozen leaves 

 are considerably above those obtained by plotting the 

 freezing-points of sap from untreated leaves. It must, 

 however, be borne in mind that the determinations are 

 only comparable in a general way, since they were carried 

 out in different years. 



On the whole it may be said that, though the daily fluc- 

 tuations are most important, yet the osmotic pressure 

 does rise with the age of the leaf. This increase is largely 

 due to the accumulation of electrolytes, as shown by con- 

 ductivity measurements. Thus, while the leaves of Syringa 

 attain their highest pressures when mature and under 

 favourable conditions for photosynthesis viz., about the 

 months of August or September in evergreens two pressure 



