OSMOTIC PRESSURES IN THE POTATO PLANT 
pressure of the root sap and that frorn the leaves should be noted. His 
results were obtained by the plasmolytic method and are of value as a check 
on those of Dixon, who made only cryoscopic determinations. His trials 
were made with a long series of plants from all sorts of habitats. The 
pressure of the root sap in nearly all cases was found to be materially less 
than that of the leaves. In only 14 percent of the cases did the pressure of 
the sap from the roots even approximate that from the leaves, while in 51 
percent the pressure of the leaf sap, as compared to that of the roots, was 
in the proportion of i : 1.25; in 12 per cent it was in the proportion of i : 1.5; 
and in 23 percent it was even greater, up to i : 2. 
The potato plant seems thus far to have escaped attention. The only 
accounts of its osmotic pressures are furnished by Atkins (i), who found 
the depression in different tubers to rarige from .538 to .612 degrees, corre- 
sponding to 6.47 to 7.36 atmospheres, and by Brannon (2), who found that 
the sap from tubers kept in an ice box from October 31 to January 23 
showed a pressure of 14.51 atmospheres while the sap from tubers kept at 
room temperature from October 31 to December 5 measured 7.4 atmospheres. 
No detailed account seems to have been given, as yet, of the osmotic 
relations of any herbaceous plant throughout an entire growing season. 
The potato seems particularly adapted to such investigation because of its 
succulent leaves and stems. Furthermore, the evolution from tuber to 
sprout and then to foliage must involve osmotic changes, and it seemed 
entirely possible that the deposition of starch in the tubers and the tip-burn 
on the foliage may be related to pressure variations in the leaf or stalk cells. 
The yearly recurrence of the physiological disease known as tip-burn 
was the immediate stimulus to the undertaking of this study. Even during 
ordinary seasons, between a third and a half of the foliage in the Vermont 
fields is destroyed during the latter part of July and the month of August 
by the intense sunlight and heat. The older plants are then affected, the 
younger ones escaping until they attain a certain stage of maturity, when 
they, too, succumb. 
The formation of tubers and flowering seem to mark a sharp crisis in 
the Hfe of the plant. Attention was directed by this point by Jones (11) 
in 1903, who said that: "Reproduction by seeds is a sexual process, that 
by tubers is vegetative. Both are exhaustive of vital forces. The two 
are, therefore, in a physiological sense opposed and cannot well be carried 
on at the same time. Under the natural condition of the wild plant the 
seed precedes; with our shorter season and intensive culture we have crowded 
the two processes together until they tend to overlap. That is, we have 
forced the tuber production back into the period which in the wild plant 
is given to the production of flowers and seeds. As a result, we have, just 
after the potato plant comes into blossom, a strained and unnatural con- 
dition; a state of physiological tension, of stress between two opposing vital 
tendencies. According to the mode of its ancestors the major part of the 
