Plant Cells at Low Temperatures. 415 
Discussion. 
The changes recorded in osmotic pressures cannot be due to peculiari- 
ties of individual plants. In the case of Picea , material for each reading 
was collected from several different trees of approximately the same age, 
and in Populus the cortical tissue for each pressing was stripped from two 
or three young saplings. To obtain sufficient sap for a reading of Pyrola 
leaves from at least 50 or 60 plants had to be used, and more than that 
number were taken for Linnaea. 
In Picea canadensis the osmotic pressure rose during the late autumn 
months, fluctuated slightly at the end of the year, and then rose steadily to 
a maximum in late March of 26-91 atmospheres. By the beginning of June 
the pressure had fallen to 19-66 atmospheres, but was still higher than the 
October value of 17*13 atmospheres. The electrolytes show only slight 
changes throughout the period from December to June, although a decided 
rise began to be apparent at the end ; approximately 7 weeks after the end 
of severe frost. The variation of osmotic pressure appears to be due to 
non-electrolytes. The sugars do not appear to play the chief part in the 
variation of the non-electrolyte curve A — A e , but the rapid increase in the 
total sugars — especially maltose — at the beginning of starch formation is 
noteworthy. The possible part played by the metabolism of fats in the 
variation of the osmotic curve will be discussed in a future paper. 
In Populus tremuloides two maxima are observed in the osmotic 
pressure, but, as in Picea , the pressure falls decidedly as the summer condi- 
tion is reached. The difference between the maximum and minimum 
osmotic pressure is only about one-half what it is in the leaves of Picea. 
Here again, the variation in pressure is due chiefly to non-electrolytes, 
although the electrolytes show a slight increase from March onwards. 
In Linnaea borealis the maximum osmotic pressure is attained about 
the third week in March and falls rapidly until June, and the electrolyte 
curve shows a decided rise at the end of the observations. It was impossible 
to avoid using a number of young leaves for the reading on May 29, and 
the small osmotic pressure and increase in the electrolytes may be due to this. 
Pyrola rotundifolia , unlike the other examples, shows a fairly steady 
decrease in osmotic pressure from December to June, and the electrolytes 
remain fairly constant throughout the season. The same fall in the sugar 
content occurs as the spring is approached, but commences somewhat 
earlier than in the other plants. 
Summary. 
1 . Osmotic pressures, electrical conductivities, proportions of electrolytes 
and non-electrolytes, and the amounts of sucrose, maltose, and glucose have 
been determined in the leaf tissues of Picea canadensis , Linnaea borealis , 
Pyrola rotundifolia , and the cortical tissues of Populus tremuloides at 
intervals from the autumn until the summer. 
