THE RELATION BETWEEN THE DENSITY OF 
CELL SAPS AND THE FREEZING 
POINTS OF LEAVES.2 
WILLIAM WOODWARD OHLWEILER. 
INTRODUCTION. 
The optimum temperatures for plant growth vary greatly 
for different regions, and the ranges of variation for differ- 
ent plants are almost as varied as the plants themselves. 
Where plants are grown under glass it is necessary to vary 
the temperature according to the requirements of the plants 
to be grown, as is shown by the use of tropical, subtropical, 
| stove, temperate, and cool houses. Some plants are closely 
limited to these temperatures, while others seem to be more 
or less cosmopolitan. However, for the species and varieties 
of plants native to any one region, there is this same varia- 
tion in the temperature requirements, some seeking the 
cool, wet bogs, and others the dry, sunny slopes. We have, 
in any region, plants that are very susceptible to cold 
periods, and others that are not. Late spring frosts destroy 
some crops with apparent regularity, while others remain 
unharmed. Why is there this difference? It could hardly 
be hereditary, since long years of acclimatization would 
have eliminated the weaklings. The structure of the cell 
walls might determine whether or not cells of certain plants 
would freeze. However, it is not the freezing of the cell 
wall which kills plants, but the freezing of the cell con- 
tents. It is more or less true that the cell walls may have 
some effect in retarding or preventing the freezing of the 
— contents, but this, when we consider the small amount of 
tissue involved, would have but little influence upon the 
1 A thesis presented to the Faculty of Washington University in 
candidacy for the degree of Master of Arts, June, 1911. 
(101) 
