OSMOTIC CONCENTRATION OF LEAF SAP 271 
taken to the laboratory for preliminary freezirig* in order to 
facilitate the extraction of samples of sap really representative of 
the whole mass of tissue, as demonstrated by Dixon and AtkinsT 
and confirmed by experiments made subsequently by ourselves. 
The freezing-point lowering of the expressed sap, cleared by 
centrifuging at high speed,§ was determined by the use of a 
mercury thermometer graduated in hundredths of degrees in 
divisions sufficiently large to permit of estimation to thousandths 
of degrees. The freezing of the sap sample was effected by 
bubbling a dried air current through carbon bisulphide or ether 
in a Dewar bulb. 
The table of constants contains the depression of the freezing 
point in degrees centigrade, A, corrected for the influence of under- 
cooling, and the corresponding values of the osmotic pressure, P, 
from a published table. || 
Specific conductivity at 30° C., x, was determined in a Freas 
conductivity cell by means of the ordinary Wheatstone bridge 
of the physiological laboratory. The cell was standardized with 
N/10 KCI, considered as having a specific conductivity of 0.01412 
at 30°. 
Freezing point lowering, A, and specific donchictivity, K, are 
the two directly determined constants. Osmotic concentration 
or osmotic pressure, P, has been determined from A, and included © 
in the table, partly because it is easier for some botanists to think 
in terms of atmospheres than in those of freezing point lowering, 
and partly because this is necessary for the purpose of ascertaining 
whether the increase of osmotic concentration with increase of 
height of insertion is at the rate which would be expected if this 
* Gortner, R. A.,& Harris, J. Arthur. Notes on the technique of the deter- 
mination of the depression of the clay point. Plant World 17: 49-53. 1914. 
t+ Dixo Atkins, W.R.G. Osmotic pressures in plants. I. Methods 
of saapend Bs from plant organs. Sci. Proc. Roy. Dublin Soc. N.S. 13: 422-433, 
I9gI n Notes, Bot. Sch. Trin. Coll. Dublin 2: 1 54-163. 1913. 
ner, R. A., Lawrence, J. V., & Harris, J. Arthur The extraction of sap 
from pee tissue by pressure. Biochem. Bull. 5: 139-142. 19 16. 
pte me one the oo cannot be wae cleared by centrifugi 
|| Hi , & Gortner, R. A. Note on the pinay of the osmotic 
pressure pl eo yakoariea saps from the depression of the freezing eee with 
a table for the values of P for A = 0.001° — 2.999°. Amer. Jour. Bot. 
IQr4. 
