1913] BRANNON—OSMOTIC PRESSURE 437 
May 5 and July 1, and a consequent concentrating of the solutes 
in the sap which was expressed with great difficulty from the old, 
wrinkled potatoes kept 8 months in dry sand at room temperature. 
In the last series of readings a comparison of filtered and unfil- 
tered sap was not made in every case, but whenever the test was 
conducted the unfiltered sap showed somewhat greater pressure 
than the filtered. 
The fact that the larger potatoes gave a higher osmotic pressure 
than smaller ones seems to hold only between those within a given 
variety. At any rate, in this last set of readings the Snowflake, 
which is a much smaller potato than the Dakota Red, gave a higher 
pressure. However, it was found that a small Dakota Red gave 

a b 
Fic. 4.—Tissue of potatoes 140 days after test began: a, Dakota Red potato at 
room temperature, with thick walls, 136; }, ice box potato, with thin walls, X 136. 
a lower pressure than a large Dakota Red kept under the same 
control conditions, and likewise that a small Snowflake gave a lower 
pressure than a larger Snowflake when they had been kept under 
the same control conditions. 
It was observed in macerating the potatoes used in the readings 
recorded in this paper that the tissues of the ice box potatoes became 
increasingly more brittle with the lengthening of the experiment. 
The supply of free starch became reduced and the proportionate 
amount of water seemed to be increased. The room potatoes 
tested at the same time in the control work were found to have 
an increasing toughness as the time of the experiment extended. 
This result was clearly related to the cell-wall structure. The 
accompanying drawings (fig. 4) indicate why the tissues of the ice 
box potatoes broke down so much more readily under the macer- 
