Dixon and Atkin.s — On Osmotic Pressure in Plants, 6fc. 309 



Another point which stands out clearly from the observations is the 

 contrast offered by the sap of the roots to that of the leaves. In Syringa 

 the pressure of the sap of exposed leaves was found to vary from 14 to 24 

 atmospheres, while that of the roots lay between 4 and 6 atmospheres. In 

 Eucalyptus the osmotic pressure of the leaves ranged between 6-1 and 8-4 

 atmospheres ; that of the roots was 5'3 atmospheres. 



Large diifereuees were observed in the mean molecular weight of the sap 

 of the roots of Syriuga, viz. 167 to 254. The former of these was obtained 

 at the end of September, and tlie latter towards the end of October. 

 Possibly the lower figure represents the normal mean molecular weight of 

 the solutes of the root-sap, while the higher figure is attained only when 

 large quantities of carbohydrates are being transported from the leaves into 

 the roots, e.g., previous to the shedding of the leaves. In succeeding 

 observations the dissolved substances in the sap of the root showed signs 

 of gradually regaining their original low molecular weight. The second 

 change may have been brought about by the deposition of starch in the 

 roots, and tlie consequent reduction in the amount of dissolved sugars of 

 high molecular weight. 



It may be noted that the lowest osmotic pressure observed so far by this 

 method was in the sap of the leaves of Chamaerops humilis, viz. 3 '79 atmo- 

 spheres. This sap is also remarkable in containing dissolved substances 

 having the highest mean molecular weight observed, viz. 323. The sap of 

 the three monocotyledons examined was remarkably clear, being almost 

 colourless, and strikingly different from the dark sap of many of the other 

 plants. 



Only two cryptogams have been examined up to the present. The 

 osmotic pressure of their sap is in no way remarkable. The mean molecular 

 weight of the dissolved substances of Bquisetum is very low, viz. 122. This 

 is the more remarkable, seeing that the sap was obtained from insolated 

 plants, and there had been nine hours' sunshine on the previous day. Possibly 

 the observation may indicate that a sugar of low molecular weight, such as 

 glucose, is formed in the early stages of the assimilatory process. 



Glancing over the list of determinations, it will be seen that in several 

 instances the osmotic pressure of the sap of the leaves investigated was very 

 high, viz. : — 



Catalpa bignonioides, 

 Syringa vulgaris. 

 Magnolia acuminata, 

 Fraxinus excelsior, 

 Fraxinus oxyphylla. 



22'9 atmospheres. 



26-8 



22-3 



25-2 



24-0 



