256 
Journal of Agricultural Research 
Vol. XXVI, No. 6 
The pressure developed in the young tubers in the greenhouse is like 
that in tubers grown out of doors. The average for the young tubers for 
both years is 6.60 atmospheres in the greenhouse plants of all sorts, 
compared with 6.30, 7.08, 7.52, and 6.18 atmospheres in new tubers from 
the field in the four years during which the work was carried on. The 
very dry condition of the soil in the benches undoubtedly helped to raise 
the pressure in the tubers grown in such soil. 
SUMMARY 
(1) The general results confirm those obtained in 1918; the growing 
portions, usually the young leaves and branches, have a higher osmotic 
pressure in their cell sap than in the sap of the old leaves, stems, or 
new tubers. 
(2) The growth of the top leaves may be checked, however, and the 
lower (older) leaves may show a very much greater pressure. 
(3) The stems usually show the highest pressure during the height of 
the activity of the plant (July 15 to August 20), apparently because of 
the presence of a high percentage of reducing sugars. 
(4) Potato plants taken from fields in one of the best potato regions in 
the northern States (Randolph, Vt.), even at the height of the plant 
activity, showed superior pressure in the young leaves and shoots and 
the plants were apparently still growing at that time, without any signs 
of tip burn. 
(5) Plants grown in tiles under a cloth shade have less osmotic pressure 
in the juices of the foliage parts as compared with plants grown in tiles 
in the open; the pressure in the stems and tubers is about the same, 
however. 
(6) No differences could be detected in the juices from the early and 
late varieties. Weather seemed to have more effect than variety. 
(7) Mosaic plants have a higher osmotic pressure in the leaves than 
healthy ones, but this does not seem to be true of the stems. 
(8) The osmotic pressure is greater in some years than in others, 
depending on the weather; a wet year lowers the pressure while a dry one 
raises it, especially in the new tubers. 
(9) The pressure varies much more between different parts of the 
potato plant than it does between those of the artichoke or dahlia; the 
potato seems to be in a state of unstable equilibrium. The older leaves 
of the artichoke may have a very high pressure, but the dahlia seems to 
be quite constant throughout, even in very warm, clear sunshine. 
(10) Potato plants grown in the greenhouse never developed a superior 
osmotic pressure in the stems; the pressure in the leaves was always much 
higher. The osmotic pressure in the new tubers was about the same as 
that of field-grown plants, probably because of the rather dry condition 
of the soil. 
