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B. F. LUTMAN 
the stalks the increase was only 1.38 atmospheres or 17 percent. On 
August 27 a similar experiment had been made on leaves removed from the 
plant and allowed to transpire for 2 3/^ hours. The increase in this instance 
was 1.6 atmospheres or about 18 percent. 
The young Green Mountain plants, growing since about August 7, had 
developed tubers a centimeter or more in diameter in about seven weeks. 
The leaves and stalks exhibited about the same pressure, while that of the 
tubers was rather higher than usual and that of the seed piece was very low. 
The large amount of sugar present in all the organs was conspicuous and is 
comparable to that in the plants, used on July 20, 22, and 23, which were in 
about the same stage of development. The increase in the osmotic pressure 
in all parts of the plant, too, shows that the soluble materials at this stage 
of growth had about reached their maximum. 
The only other observations on normal potatoes were made on Septem- 
ber 28, on some old McCormick plants that had started to put out new 
foliage. The new leaves at this date contained a sap with an osmotic 
pressure 0.73 atmosphere higher than the sap of the stems. On September 
19, the reverse had been true. 
Shading the potato plant diminishes the osmotic pressure of the sap of 
the leaves and stalks. The pressure in the leaves of a normal young plant 
on September 9 was 6.99 atmospheres, while that found in a plant shaded 
for 48 hours was only 6.29 atmospheres. In the stalks of the same plants, 
the pressures were 6.76 and 6.48 atmospheres. Shading, therefore, by 
lowering the osmotic pressure would enable parasitic fungi more readily to 
obtain their food material and to increase their rate of growth. Dark, 
cloudy, rainy days would be very effectual to this end, since increased mois- 
ture seems to lower the pressure, as is clearly shown by the observations on 
July 27 and 29 taken before a rain, and on July 30 and 31 after a rain. 
The relation of lowered osmotic pressure to the spread of epidemics due to 
fungi has not been heretofore recognized clearly, although Dixon afid 
Atkins (5) have already pointed out that shading had this effect on the 
leaves of Syringa : 
Covered, 2 days, 21.63 atmospheres 
Exposed, 2 days, 24.57 
Covered, 3 days, 19.03 
Exposed, 3 days, 20.00 
Covered, 7 days, 15-97 
Exposed, 7 days, 19.12 
The effect of the weather was also noted by them on the same plant: 
Leaves, gathered after a dark day, no sunshine, 16.26 atmospheres 
Leaves, gathered after a bright day, 9 hours' sunshine, 22.40 atmospheres 
Leaves, gathered after a bright day, 7 hours' sunshine, 20.40 atmospheres 
The checking of epidemics of Phytophthora injestans by hot, dry weather 
is usually ascribed to a "drying up" of the fungus in the leaf. This is 
