Dixon and Atkins — On Osmotic Pressure in Plants, SfC. 295 



of the water-tracts was not tlie controlling factor of the pressure ; accordingly 

 some other cause for its variation must be sought. This cause seems to be 

 piincipally the fluctuations in the sugar-content of the leaves due to difference 

 in illumination. In expt. 21 and 22, the leaves of which are from shaded 

 positions, smaller pressures are found than in expt. 24, which was performed 

 on sap from leaves in a sunny position, while expt. 23 on leaves coming 

 from the outside of the crown facing a clear north sky, and being 

 consequently better illuminated tlian the other two samples, 21 and 22, has 

 a higher pressure than they, though considerably lower than in 24. 



The effect of exposure to the sunlight on the osmotic pressure is 

 illustrated by a great number of experiments. Nos. 2v) and 30 show the 

 difference between the pressure in leaves on the north and south side of a 

 shrub of Cerasus Laurocerasus. 



Tablk Y1. 



Cerasus Laurocerasus : leaves. 



Possibly here tlie high mean molecular weight may in part be due to the 

 presence of small quantities of the glucoside amygdalin. 



After a sunny day the pressure of leaves rises. It falls at niglit, as may 

 be seen below iu the case of Si/riiiga vulgaris. 



Table VII. 



SyriiHja vulgaris : leaves. 



Both samples of leaves were taken from tlie same position on the same 

 shrub, and so, if they had been taken at the same time, should have hud 

 equal pressures, as was shown earlier. 



