Nov. io, 1923 
Osmotic Pressure of the Juices of the Potato Plant 251 
SUMMER OF 1920 
The difference in the date of the death of the plants in early and late 
varieties must lie in some factor internal to the plant itself. Differences 
of osmotic pressure in the various parts of the plant at various stages 
of growth might give some clue to the early loss of foliage and ripening 
of the so-called early types. Very early and very late varieties of 
potatoes were planted, therefore, and tests were made on them at various 
stages during the growing season. The McCormick Pink and White 
variety (reputed to be very resistant to hot weather and tip bum), 
from which some differences in osmotic pressure might have been 
expected as compared to early varieties that succumb prematurely to 
tip bum, was a disappointment. The plants very early showed pro¬ 
nounced symptoms of mosaic and only one cryoscopic reading was taken 
of the juices. 
The season was a favorable one, as it presented a contrast to the two 
preceding years in the weather and in the time of the appearance 
and the amount of tip burn. The official charts do not show any great 
contrast in temperature, sunshine, and rainfall, but the effect on the 
plants was not the same, owing to the fact that much haze and humidity 
nearly always tempered extreme periods of heat and sunshine. The 
rainfall during June and July was almost 2 inches above normal, while 
that of August was 1.64 inches below normal. The rain was well dis¬ 
tributed throughout the month, however. The plants grew large and 
succulent and tubers developed rapidly. The foliage was so extreme 
and covered the ground between 3-foot rows so densely that it was 
impossible to avoid stepping on the vines. Tip burn was practically 
absent until late in the season, after which it advanced fairly rapidly, 
owing to the rank growth. The tip bum was first noted about August 
19, almost a month later than it usually appears here. 
The results from the cryoscopic readings of the juices of the potato 
and other plants in general confirm in the following particulars those 
taken during 1918: 
1. The young growing portions of the plant contained juice with a 
somewhat higher osmotic pressure than the stems and a very much 
greater pressure than the old seed piece. 
2. The pressure in all parts of the plant is comparatively low during 
the early growth of the plants, rising to a maximum during August and 
dropping away during September. 
3. The marked differences shown in the freezing points of the juice 
from various parts of the potato plant do not seem to be shown by the 
dahlia or artichoke. 
The results in 1920, however, do not agree with those of 1918, since in 
1918 the stems developed much higher osmotic pressures than the 
leaves just at the time that foliage growth had ceased and tuber forma¬ 
tion begun. The foliage juice at all times in 1920 showed a greater 
pressure than the juice from the stems, although there were times when it 
was almost the same. 
A few comments on the readings and tables, to bring the facts together, 
will make the relations a little clearer. 
Dibble's Russet (a late variety) and Early Rose are the only varieties 
of which a fairly complete record was obtained during the critical por¬ 
tions of the growing season. Tables VI and VII summarize the readings. 
