Atkins — Osmotic Pressures of some Plant Organs. 467 



The Brassica Rapa used was at first the white variety. Two roots which 

 were kept exposed in the laboratory for a week increased in pressure by 

 evaporation, while there was no decisive alteration in the value of M. The 

 Swede turnip used was a large one ; and all the determinations were made 

 on it. After being freely exposed to the air in the laboratory for a few days 

 its osmotic pressure was found to have risen from 10*70 to ]2'36 atm. This 

 agrees with Sutherst's result of about 12 atm. Further keeping in a closed 

 vessel in the dark did not increase the pressure, but actually lowered it 

 slightly. The lowering was probably only apparent, as the outer parts used 

 in the second determination were, owing to evaporation, drier than the inner. 

 Keeping in a closed vessel tends to equalize the water content, and so 

 apparently lowers the concentration of tlie outer parts. The mean molecular 

 weight showed no considerable variation, but was seen throughout to be higher 

 than that of the white variety. 



Tubers of Helianthus tuberosus behaved rather differently, as the pressure 

 was found to rise from less than 14 atm. to over 18 on keeping in a closed 

 vessel in the dark for a fortnight. These results do not agree with those of 

 H. Fischer and Copeland, who found less than 3-5 atm. pressure in these 

 tubers.' The values of M obtained, 371, 394, 370, agree well, and are 

 comparable to the values found by Maquenne, 445 and 306 in germinating 

 rye and peas.^ 



It was noticed that on boiling the juice from S. tuberosus the quantity of 

 albuminous substances precipitated was very considerable. 



The tubers of Solanum tuberosum were also examined when fresh, and at 

 intervals after keeping in a closed vessel in the dark for nearly a month. 

 Eeference to the table shows that there was no regular variation during that 

 period, the osmotic pressure of the various tubers fluctuating between 6-47 

 and 7-36 atm. The mean molecular weight also ran irregularly from 140 to 

 159. Thus, though shoots were just being formed, there was not sufficient 

 metabolism to alter the character of the tuber as a whole. 



[Table of Eesults. 



> Quoted from Jest's Plant Physiology, p. 419. Eng. Trans. 

 ^L. Maquenne, Comptes Eendus, torn, cxxv., p. 576, 1897. 



