Dixon and A'J'kins — Variations in Osmotic Pressure, ^-c. 233 



The figui'es in this table may be summarized as follows ; — 



Leaves on ultimate growth : 



Ultimate 3 : mean = 0738°. 



max. = 0'916°. Jan. 2, 1911. 



Leaves on antepenultimate groioth : 



mean = 0-978°. 



max. = 1-061°. Nov. 21, 1910. 



min. =0-901°. Feb. 10, 1911. 



Roots : . . . mean = 0-670°. 



max. = 0-903°. Aug. 28, 1911. 

 min. = 0-365°. Nov. 18, 1910. 



Figs. 1 and 2 are graphs constructed from Table III. 



A glance at these numbers and curves shows tlie extremely erratic manner 

 in which the freezing-point (and with it the osmotic pressure) varies. 



In the first place it may be noted that the intensity of illumination does 

 not define the rises and falls of the curves. Except in the case of the 

 penultimate leaves (fig. 1) where the curve is raised by one erratic observa- 

 tion, tlie lowest average osmotic pressure is found in the summer months. 

 Again, at the end of the month of May, 1911, in which there bad been 235 

 hours' sunshine, and where we might have expected a concentration of 

 dissolved substances if their accumulation depended on the intensity of 

 illumination, the depression of freezing-point of the sap from the leaves of 

 the ultimate growths was only 0-718° and that of the penultimate growths 

 only 0"811, while at the end of the previous December, in which there had 



ultimate growths which, was not divided, as in the other cases, into ultimate and penultimate leaves. 

 Hence it is reasonable to suppose that the ultimate three leaves of these shoots had a depression of 

 freezing-point less than 0-730°, and the penultimate three had one of more than O'TSO". 



' This mean is obtained by omitting the high observation on June 13, which, as already explained, 

 should be treated as belonging to the leaves of the penultimate growth. 



