Nov. io, 1923 
Osmotic Pressure of the Juices of the Potato Plant 255 
Table X .—General average on juices of Green Mountain variety during four years 
Portion of potato used. 
1918 
1919 
1920 
1922 
Young leaves. 
7- 54 
6- 59 
8. 74 
6. 30 
. O. OO 
10. 41 
7 OA 
Old leaves... 
• y* ? 
10. 02 
IO. 20 
8.97 
7- 5 2 
9. 19 
8 . 56 
6. 18 
Stems. 
AV/ . 
O. I A 
New tubers. 
y* XA t 
7. 08 
It will be noticed that the readings for 1918 and 1922 are lower than 
those of 1919 and 1920, and that this difference is even reflected in the 
atmospheric pressure in the new tubers themselves. The high pressure 
in the stems (as compared to that from other portions of the plant) 
during 1918, and that in the old leaves during 1922, are other peculiar 
features. 
POTATO PLANTS GROWN IN THE GREENHOUSE, 1921 AND 1922 
The almost complete freedom of greenhouse grown plants from physio¬ 
logical tip bum and the fact that such plants will continue to remain 
green and to continue growth ought to make the osmotic pressures of 
interest as compared with those from the field. 
The osmotic pressure in the young sprouts that come from the seed 
piece is generally higher than that of the seed piece itself, but an excep¬ 
tion was to be noted in the Burbank and Early Rose on March 22, 1921, 
and in the Green Mountain on April 18. The osmotic pressure in the 
seed piece drops away rapidly as the stored starch is used by the plant, 
but an exception again occurs in the Green Mountain seed pieces on 
April 27, and the Early Rose on June 27, where the pressure continued 
high. The dry condition of the soil may explain this high pressure, as 
the water may have passed out of the seed piece, gradually leaving a 
more concentrated solution behind, or the plant may have been unable 
to supply water to the seed piece to remove the sugars which went into 
solution. The soil was very dry and probably drier in some beds than 
in others. 
The roots examined March 22, 1921, had the lowest osmotic pressure 
of any portion of the growing plants, as was to be expected. 
It may be noted generally for both years that the foliage maintained at 
all times a greater osmotic pressure than any other organ of the plant. 
In general, the pressures in younger tip leaves were greater than in the 
older leaves, but there were exceptions. The juice of the stems, however, 
at no time approached that of the foliage, and in this relation we have 
something radically different from the conditions in plants grown in the 
field where, at some time during the growing season, the stems always 
developed a higher pressure than the leaves. The high stem pressures 
came usually at the periods of tip burn, and the author is inclined to 
associate the two phenomena. While the plant is very active photo- 
synthetically the sugars that are so rapidly formed are stored temporarily 
in the stem to the detriment of the leaves of the plant. Shaded plants, 
greenhouse plants, and the plants from Randolph, Vt., did not show 
this abnormally high stem pressure and did not show any amount of 
tip burn. The two sets of facts may be unrelated, but the common 
occurrence is at any rate highly suggestive that they are connected. 
