Dixon and Atkins — Osmotic Pressures in Plants. 



439 



is less than that iu the leaves. Of course, the actual figures obtained by the 

 new metliod are higher than those obtained by the earlier one. It also 

 appears that this gradation from below upwards may be, in some cases, 

 extended, for we find that tlie finer roots of Ilex have a lower osmotic 

 pressure than the thicker ones, and the bases of the leaves of Iris have a 

 slightly lower pressure than the tops. 



With regard to the seasonal elianges iu Hedera, Ilex, and Syringa, studied 

 in our former papers, we have not yet had time to measure the variations 

 over any prolonged period, but so far as the present results go tliey confirm 

 in a general way those obtained before. It is possible that the fluctuations 

 in the yearly curve of pressures will be less irregular now that tlie uncertainty 

 in the manner of extraction is removed. 



The specific conductivity measurements of the saps examined are of the 

 same order as that of N/25 or 0'3 per cent. solution of potassium eliloride. They 

 range between 112 x lO"'^ and 1141xlO"^ With regard to the relative magni- 

 tudes some very surprising results have been obtained. Fruits were found to 

 have quite low values in spite of their known large content of malic, tartaric, or 

 citric acid. These, it is true, are not strongly dissociated acids, and evidently 

 their effect is not nearly so marked as that of salts, organic and inorganic, ■ 

 where they occur, for the salts of even very weak acids are strongly dissociated. 

 Heald, as was noted before, found a remarkable parallelism between the 

 conductivity of tiie sap and that of the dissolved ash from the same organ. 

 Tills establishes the small influence of organic salts and acids on the 

 conductivity, and gives indirect evidence, as it seems to us, of the smallness 

 of the error due to viscosity. 



The maxima and minima of the observed specific conductivities for the 

 sap of different organs appear from the following abstrtict : — 



