IMPORTANCE OF WATER 



127 



The tube frozen without the addition of sugar will show a flaky- 

 precipitation of the coagulated protein (Fig. 48). 



It is of interest to note that the sugar content in dormant plants 

 is not constant. It increases with lower and diminishes with higher 

 temperatures. The frost resistance of plants varies accordingly. 

 It was noticed, long ago, that winter cereals when grown in warm 



Fig. 48. — Protective influence of sugar in freezing expressed plant juice {after 



Newton) . 



greenhouses are easily killed by freezing, while those grown in the 

 open show a high degree of hardiness. Even a prolonged thaw 

 decreases perceptibly the cold resistance of plants. On the other 

 hand, if plants grown in a warm place are kept for several weeks 

 at a temperature but slightly above freezing, they may be hard- 

 ened and may withstand a mild frost. Such hardening is always 

 accompanied by an increase of the sugar content in the cells of 



