Dixon and Atkins — On Osmotic Pressure in I'lants 6,-c. 307 



favourable to assimilation and transpiration, be considerably raised appears 

 probable from experiments 38 and 39. The former shows that the pressure 

 of the leaves when gathered was 17'12 atmospheres, while the latter shows 

 that, by exposing some of the same sample of leaves to very favourable 

 conditions, this pressure was caused to rise to 25'68 atmospheres. A similar 

 rise is shown in experiments 33 and 34, where assimilation and transpiration 

 liave caused a rise from 16"26 to 24'07 atmospheres. These observations 

 would lead us to believe that tlie osmotic pressure of the sap of leaves of 

 Si/riiiga vulgaris might easily rise to 30 atmospheres or more. 



Turning now to the determinations of the mean molecular weights of 

 the dissolved substances, they are seen to vary from 156 to 273. The great 

 majority lie about, or over, 200. It is remarkable that, while screening the 

 leaves from light always reduces the osmotic pressure, it often has little or 

 no effect on the mean molecular weight. In fact, the highest mean molecular 

 weight observed in the series on Syriiiga was in the sap of leaves which 

 had been cut off from light for three days (experiment 46). On the other hand, 

 the lowest mean molecular weights are also found after the leaves have been 

 out off from light (cp. experiments 37, 63, and 55). As was pointed out 

 before, we must probably look to the fluctuation of the sugar-content of the 

 cells as the chief cause of variation in the mean molecular weight of the 

 dissolved substances. When the percentage of the disaccharides rises, the 

 molecular weight will more nearly approach 342, the molecular weight of 

 saccharose and maltose. A rise in the concentration of the monosaccharides, 

 dextrose, and levulose, having a molecular weight of 180, will not necessarily 

 tend to bring up the mean molecular weight of the solutes above that figure. 

 The formation of organic acids will not tend to bring the mean molecular 

 weight above 200. The fact that saccharose is known to be formed 

 during assimilation is sufficient explanation of the observations showing 

 that insolated and well-illuminated leaves have a sap with a high mean 

 molecular weight. The general high mean molecular weight of the 

 solutes in the sap of leaves screened from light must be explained by 

 assuming that some disaccharide is produced in them or is transported 

 to them. From Brown and Morris's elaborate work it is certain that 

 maltose is developed by the action of diastase on stored starch. They 

 show reason to believe that carbohydrates are transported in the form of 

 levulose. This would seem to cut out translocation as the direct means of 

 maintaining the high molecular weight, and would seem to show that the 

 hydrolysis of starch explains the high molecular weight found in these cases. 

 The very low values exhibited in some of the experiments on screened 

 leaves are also readily explained by the exhaustion of stored carbohydrate, 



SCIENT. PROC. E.B.S., VOL. XII., NO. :XXV. 3 



