Nov. io, 1923 
Osmotic Pressure of the Juices of the Potato Plant 253 
Table VIII .—Effect of shade on osmotic pressures in the juices 
(Green Mountain, Aug. 25, 3 p. m.) 
Portion of plant used. 
Plants in 
open. 
Plants under 
shade. 
Young leaves .*.. 
I. 061 
. 964 
•947 
•733 
0.965 
.828 
• 763 
. 681 
Old leaves. 
Stems... 
Tubers . 
The Green Mountain plants used for the above experiment were grown 
in large tiles, half of them shaded with a heavy cotton cloth which cut 
off at least 50 per cent of the light. A comparison of the readings will 
show that while the plants grown in the open exhibited a greater depres¬ 
sion in the young leaves and tubers, the greatest difference was shown 
in the readings obtained from the old leaves and stems, especially the 
latter. The older leaves on the open-air plants were beginning to suffer 
from tip burn at the time of this experiment in spite of the very great 
osmotic pressure, while those in the shade were practically untouched. 
The high pressure in the juice of the open-air stems is probably the result 
of the very much superior starch assimilation which the leaves of these 
plants were carrying on. 
A few observations were made on plants other than potatoes, for pur¬ 
poses of comparison. Lamb’s quarter, on July 23, showed about the 
same relations as to osmotic pressure as did the potato plant. The pres¬ 
sure at this time, however, was above that obtained on July 14 and 27 
for potatoes. Lettuce, examined on July 28, gave comparatively low 
pressures; the plants were still succulent and tender, with only a very 
short stalk. Sap from tomato plants examined on August 18 showed 
much less osmotic pressure than sap obtained from the various parts of 
the potato plants used on the same day. However, these particular 
potato plants were beginning to show tip burn. These readings are com¬ 
parable to those obtained in 1918 from tomato, but the plants in that 
year were taken very much later and after cold rains, and were conse¬ 
quently much lower. Lettuce, examined August 19, was tried again 
after the plants had begun to lengthen into flower stalks, but no marked 
difference is to be noted in the stem and leaf juice atmospheric pressure, 
although it is much higher than it was at the time of the earlier reading 
on July 28. 
Dahlias and artichokes should show some of the same internal physio¬ 
logical conditions as the potato plant, but it must be remembered that 
the tubers from them are swollen roots, and not enlarged underground 
stems. The pressures in the dahlia plant, examined August 27, are 
practically at an equilibrium, but the new tubers show the least pressure. 
The pressure in this plant seems to be much less than it was in the Early 
Rose and Green Mountain potato plants taken from adjoining rows. 
The readings on the artichoke plants on September 5, taken on a hot day 
and after the plant growth had ceased, showed very much higher pressures 
than those of the dahlia plant—in fact, they are comparable to those 
from the potato plant. This apparent anomaly shows that the juice of 
the older leaves has higher pressure than that of the younger leaves and 
tips of the stems. The juices of tubers have a comparatively high pres¬ 
sure, but the lowest of any of the juices in the plant. 
