OSMOTIC PRESSURES IN THE POTATO PLANT 
probably literally true. The fungus may be able to withdraw food ma- 
terials from the leaf when the pressure is only about 6 atmospheres, as on 
July 31, but when this pressure rises to 9 or 10 atmospheres, as on August 
8, the parasite may itself lose all its water and be unable to recover. 
Cryoscopic readings of the sap from plants badly affected by mosaic 
were made on September 9 and on October i. The leaf sap of the mosaic 
plant recorded a pressure of 7.83 atmospheres, while that from a normal 
plant growing near it showed only 6.99 atmospheres. The mosaic leaves 
also contained an unusually high percentage of reducing sugars. The 
pressure in the normal leaves on October i was 9.39 atmospheres, while 
in the mosaic leaves it was 10 atmospheres. The much higher percentage of 
cane sugar should also be noted. On the other hand, the sap of the normal 
stalks exhibited a pressure of 8.30 atmospheres while that of the mosaic ones 
showed only 7.60 atmospheres. The higher pressures in the mosaic leaf 
sap seem to be due to the presence of abnormally large amounts of sugars. 
This may indicate an inability on the part of organs so affected either to 
transform or to transport their carbohydrates. 
Comparison of the potato plant with nearly related plants, such as the 
tomato, or with vegetables that deposit their carbohydrates in enlarged 
roots, such as the carrot or the beet, ought to throw some light on the pres- 
sures in storage organs or fruits. The juices from the tomato plant tried 
on September 24 recorded about the same pressures in leaves and stalk. 
Growth at this time had ceased. The low osmotic pressure in the green 
fruit is peculiar but corresponds almost exactly with the pressure in the 
fruit from the potato on September 19. 
The ripening of the tomato fruit increased its osmotic pressure 0.76 
atmosphere as shown on September 24 and verified on October i , with very 
ripe fruit. Unfortunately no tomato plants were used for osmotic pressure 
experiments during the very hot weather in August, but the probability 
is that the stalk sap would have shown a higher pressure than the leaf sap, 
as was the case with the potato in that period. Corn, on July 31, recorded a 
pressure of 6.85 atmospheres in the leaves and 7.52 atmospheres in the stalks, 
while beans gave a pressure of 9.54 atmospheres in the leaves, 10.94 atmos- 
pheres in the stalks, and only 8.62 atmospheres in the pods and beans. 
Carrots, on September 28, exhibited 11.80 atmospheres pressure in the 
sap from the leaves and 9.77 atmospheres in the sap from the roots them- 
selves. 
Garden beets, on September 28, had 8.83 atmospheres pressure in the 
leaf sap and 11. 81 atmospheres in the beet root sap. The 7.5 percent of 
cane sugar in the root explains the unusually high pressure. Sugar beets 
and sugar beet leaves were tried on October i. The inner leaves were still 
growing and their sap had a pressure of 11.42 atmospheres while the outer 
ones gave only 9.65 atmospheres. Another lot of beets recorded 12.21 
atmospheres for the leaves and 17.18 atmospheres for the roots. The 
