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F. J. Lewis and G. M. Tuttle 
electrical conductivities and sugars of the sap. Records were also 
made of the structure of the cells in the leaves of winter evergreens 
during the winter period. 
The present paper deals with the same features during the spring 
conversion from the winter condition, particularly with regard to 
the causes affecting conversion. 
Meteorology 
A brief description of the conditions of the winter may be of 
interest. The daily temperatures from January i to April 30 are 
shown on page 227. The winter of 1919-20 was longer than usual. 
Snow and zero (F.) weather began in the third week of October. Long 
and frequent snowfall resulted in a covering of over 8 feet in March 
and the soil was not uncovered until the second week in April. In 
some districts the ground was still snow-covered at the middle of 
May. Winter weather conditions thus overlap the seasonal changes 
in the leaf cells of Picea during this year. This is a point of great 
interest, as the change from winter to summer conditions started 
during the cold spell early in April when minimum temperatures of 
— 14 0 F. were experienced and when the maximum was always below 
30° F. 
Measurements from the graph (p. 227) indicate that of the area 
enclosed by the minimum temperature curve only 0*4 per cent, was 
above 32 0 F. and of the maximum temperature curve only 17 per 
cent, was above 32 0 F. during the period January 1 to April 30. 
Physical Conditions of Leaf Sap during Conversion 
Some features of the differences in the cell organisation during 
winter and summer were described by us in a former paper (2). These 
observations were extended during the next year, especial attention 
being paid to the physical conditions of the cell at the time of the 
spring change. 
Readings were begun during early March and were continued 
until the end of April, the spring change having occurred about 
April 5. They are recorded in Table I. The sap after being extracted 
by the methods described in a former paper (2), was filtered, and the 
A and conductivity determined immediately after extraction by the 
Kohlrausch method. The viscosity of the sap was measured with an 
Ostwald viscosimeter and the conductivities corrected for viscosity. 
