Dixon and Atkins — Osmotic Pressures in Plants. 53 



It may be mentioned here for the sake of comparison that a— 



1 per cent, solutipn of glucose gives a depression of O106°, and a 

 1 per cent. „ ,, sucrose ,, ,, „ 0'054°. 



These figures indicate the values O'OS 1 , O08 c , and - 266° respectively for 

 A - A e in the above three experiments. As may be seen from the table, the 

 values of A - A e show that the sugar determinations are over- and under- 

 estimates in the first two respectively, and the error in each is much the 

 same. The third is an under-estimate, but the agreement is good. The 

 occurrence of maltose, as well as of hexoses, is denoted by m. Microscopic 

 examination of the osazones showed that in the first maltose and hexoses 

 were present in roughly equal quantities ; in the second the hexoses 

 predominated, and the same was true of the third. 



The table shows that the fraction of the osmotic pressure of the sap 

 attributable to non-electrolytes is greater than that due to electrolytes, but 

 the preponderance becomes more mai'ked at the higher levels. As noted in 

 the first paper (1), this demonstrates that the percentage by weight of the 

 non-electrolytes far exceeds that of the electrolytes, for osmotic pressures are 

 proportional to molecular concentrations, and the sugars have much higher 

 molecular weights than have the salts of the sap. The latter, moreover, are 

 ionised, so as to yield two or more ions, each of which functions as a molecule 

 as far as osmotic pressure is concerned. 



Furthermore, it is seen that the rise in osmotic pressure as the higher 

 levels are reached is entirely brought about by the increase in the quantities 

 of sugar present, not by the electrolytes. This must not be taken to mean 

 that no other non-electrolytes are present, but only that the agreement 

 between the values of A - A e and the results of the rough sugar estimations 

 are such as to justify one in speaking of the sugars as the controlling factor 

 in the total of non-electrolytes. 



Comparison of the figures given by Ulmus with those of Acer recorded in 



this and the first paper shows that the relatively high values for the osmotic 



pressure and sugar content of the sap are due to the vernal mobilization of 



carbohydrate reserves. The presence of maltose also points to this, as it is 



usually absent from wood-sap at other times of the year, being probably 



hyclrolysed to glucose in the living elements of the wood when secretion of 



sugar takes place at a slower rate. With regard to this secretion, or diffusion 



as it appears to be in reality, it may be recalled that Osterhout (2) has shown 



how greatly the presence of certain sugars may increase the permeability of 



protoplasm. In this sucrose has a more marked action than have other 



sugars. 



i 2 



