OSMOTIC PRESSURE IN THE CELL 145 



Here also will be found the essentials concerning the formerly much discussed 

 osmotic equivalent, and the sub-maximal osmotic powers of diosmosing substances. 



Physiological estimations. By removing the turgor and determining the 

 weight necessary to stretch a tissue to its original length, the work done in the 

 living plant by osmotic energy may be calculated, while the osmotic value of 

 a substance may be found by determining the concentration necessary to remove 

 turgidity. For various reasons only approximately accurate results are obtained by 

 these methods, but it was the high osmotic pressures found to exist in the plant 

 which afforded the stimulus to the physical researches mentioned \ 



Isosmotic determinations. The estimations of the osmotic value of different 

 substances have for the most part been made by de Vries, who employed for this 

 purpose epidermal cells with coloured sap, obtained from the leaf sheath of Curcuma 

 rubricaulis, from leaves of Tradescentia discolor, and from the petiolar scales of 

 Begonia manicata. Tradescentia discolor is especially useful, since it can always 

 be procured. In the leaves of this plant, the cells of the upper epidermis, lying 

 over and near the midrib, have comparatively the same turgidity (about 0-12 

 equivalent value *= 1-2 per cent, solution by volume of KNO S ). If preparations are 

 placed in beakers containing 5 to 10 c.cm. of fluid, a condition of equilibrium is 

 reached in from | to i| hours. By determining the degree of concentration necessary 

 to cause most of the cells to show incipient plasmolysis (Fig. 10, p. 1 16), the osmotic 

 values of non-diosmosing and non-injurious substances may be obtained to within 

 o-oi to 0-02 equivalents ( o-i to 0-2 per cent. KNO 3 ) of their actual value. 



At the commencement of plasmolysis, the osmotic pressure of the external fluid 

 is slightly higher than that of the turgid cell, even when the cell-wall does not 

 undergo any elastic contraction. If a marked contraction occurs, the decrease in 

 volume must be determined before an accurate estimation of the original turgid 

 pressure is possible. Temporary variationsof turgidity, due to transitory reactions, 

 cannot be detected by the plasmolytic method 3 . 



Changes of size and form, due to the action of the internal hydrostatic pressure, 

 may be used to determine the value of the latter : thus in split stems, curvatures 

 take place owing to the unequal tissue-tensions, and de Vries, by finding the degree 

 of concentration of a plasmolysing fluid necessary to bring about a subsequent 

 straightening, has been able to make a series of osmotic determinations (1. c., p. 484). 

 It is not necessary to make any mention of other physiological methods, or of the 

 physical apparatus which may be employed for this purpose 4 . 



readily attainable apparatus would be advantageous. It was with a similar apparatus to that 

 employed by Pfeffer that Ladenburg (Ber. d. Chem. Ges., 1889, p. 1225) and Adie (Beibl. z. Ann. 

 d. physik. Chem., 1891, Bd. xv, p. 749) worked. Certain technical details are given by Walden 

 (Zeitschr. f. physik. Chem., 1892, Bd. X, p. 700). 



1 See Pfeffer, Osmot. Unters., 1877, Preface. On such pressure determinations, cf. Pfeffer, 

 Physiol. Unters., 1873, p. 122, and Period. Beweg., 1875, pp. 105, in; Plasmahaut u. Vacuolen, 

 1870, p. 309; de Vries, Mechan. Unters. iiber Zellstreckung, 1877, p. 118, and Jahrb. f. wiss. Bot., 

 1884, Bd. XIV, p. 529. In these works additional references will be found. 



2 [Turgidity is measured in osmotic equivalents of KNO 3 , a decinormal solution (10-1 grm. in 

 a litre of water) being taken as the unit of value.] 



3 Pfeffer, Energetik, 1892, p. 228. 



4 Hamburger (Zeitschr. f. physik. Chemie, 1890, Bd. VI, p. 319) made use of the red blood- 



PFEFFER L 



