OSMOTIC PRESSURES IN THE POTATO PLANT 
Methods 
The plants were brought in from the field or garden during the summer of 
1918, and the sap was immediately extracted from the leaves, stems, or 
tubers by grinding up these organs in a small Excelsior food chopper and 
pressing the juice through cheesecloth. The requisite 12-15 c.c. needed 
were usually easily obtained as all the organs except the roots are very 
succulent. The sediment was allowed to settle and the upper portion 
decanted off. The freezing was done with the help of an ordinary Beck- 
mann thermometer and apparatus. At least two determinations were 
made in each case, and if no super-cooling appeared, the trials were repeated 
three or more times. It was impossible to obtain any super-cooling with a 
few samples of leaf juice, but enough attempts were made so that the true 
freezing point was closely" approximated. Correction of the depressions 
was made for super-cooling using the formula suggested by Harris and 
Gortner (10), and their tables have been employed in converting the depres- 
sions in degrees into atmospheres. In a number of cases, the juice was 
analyzed for glucoses (or reducing sugars) and sucrose by the gravimetric 
Fehling's method. The presence of various organic materials in the juices 
introduced disturbing factors, but the determinations at least afford some 
idea of sugar percentages. Complete analyses were made of four juices. 
The author acknowledges the help of Mr. R. L. Gale, who had general 
charge of the analytical work and who assisted with the cryoscopic readings. 
Green Mountain potatoes were used in all cases unless otherwise stated. 
Comments on Observations 
The freezing point of the tuber sap varied considerably, as might be 
expected in view of. the conditions under which the tubers were kept. 
However, in every case the pressure exceeded 7 atmospheres. These 
depressions were greater than those obtained by Atkins (i) but correspond 
very closely to those secured by Brannon (2) for other varieties of potatoes 
kept at room temperatures. The pressure in the sprouts was even more 
variable, covering a range of from 6 to 12 atmospheres. The sprouts used 
in reading no. 3 (table i) were obtained from the tubers used in reading no. 2. 
It will be seen that the pressure in the sprouts was 8.75 atmospheres while 
that in the tubers was 7.691 atmospheres. The sugars were more than 
twice as abundant in the sprouts as in the tubers and were the cause of the 
increased osmotic pressure. 
The osmotic pressure of the sap of the very young plants was not de- 
termined during June and early July. The first records were secured on 
July 18 when the plants were full grown, in bloom, and with tubers a centi- 
meter or a centimeter and a half in diameter. No tip-burn had appeared 
as yet, and the plants were turgid and healthy. The results secured on this 
particular plant indicate the greatest pressure in the stalks. The weather 
