Dixon and Atkins — On Osmotic Pressure in Plants, Sfc. 299 



From tlie third pair of observations it appears that translocation, or some 

 other process, increased tlie percentage of disaccliarldes, or possibly salts of 

 organic acids, among the solutes to more than restore the original mean 

 molecular weight, while the actual concentration of the solution has further 

 diminislied, so that the comparatively low osmotic pressure of II '58 

 atmospheres is attained. 



These experiments are interesting, showing that the external conditions 

 can bring about a great change in the osmotic pressure of leaves, viz., from 

 24'57 to 11 '58 atmospheres. 



Table XII. 



Si/ringa vulgaris : leaves. 



In experiments 37 and 38 a large difference is shown, both in the osmotic 

 pressure and in the mean molecular weight; but the sample of leaves 

 enclosed in the opaque bag grew in an overshadowed position, attached to 

 branches rising from the base of the stem. Consequently these had a low 

 osmotic pressure to start with, and possibly also a low mean molecular 

 weight. In any case the isolation of the leaves in question would greatly 

 delay the transport of substances with high molecular weight which could 

 make up for the loss of carbohydrates by respiration. It is curious that the 

 two lowest determinations of the osmotic pressure 37 and 55, viz., 11-57 

 and 11'58 atmospheres respectively— one obtained by short covering of 

 normally overshadowed leaves, and the other by a prolonged covering of 

 normally exposed leaves — ^^should so closely correspond. Possibly we have 

 here an irreducible minimum of healthy Syringa leaves. 



It has also been easy to show the marked effect of assimilation on the 

 osmotic pressure in detached leaves. For this experiment leaves gathered 

 from the same region of a tree were divided into tliree samples — a, b, and e. 

 Sample a was examined immediately. The leaves of b were set with their 

 petioles in water, and exposed to conditions favouring assimilation, viz., 

 sunshine and breeze, for an hour and a half, and were then pressed and 

 examined. Sample c was kept for two days in the dark, the petioles being 

 in water. 



SCIENT, PBGC. R.D.S. VOL. XII., NO. XXV, 3 8 



