RESISTANCE OF PLANTS 353 



shown that even these organs freeze at —15 to — 20°C. With 

 still lower temperatures, they become as brittle as glass. More- 

 over, all such external adaptations as the layer of cork covering 

 the branches and the bud scales, which, for a long time, were 

 regarded as protections against cold, in reality are unable to check 

 heat loss for any considerable length of time. The role of these 

 structures is a different one: viz., they protect against desiccation 

 during the winter, which threatens the aerial parts of the plant 

 as a result of the decreased water supply from the frozen soil. 

 Frost resistance is not a capacity of dormant plants to protect 

 themselves against the formation of ice but a property of being 

 very little or not at all affected by its formation. 



This lack of susceptibility is attained as a result of chemical 

 changes in cells. It has been noticed for a long time that during 

 the cold season the leaves of certain plants, though wintering 

 under snow, show no traces of starch. Considerable amounts of 

 sugar are accumulated instead. Experiments have shown that 

 sugar has the property of preventing the coagulation of protein 

 substances at low temperatures. It therefore may be called a 

 protective substance. This protective influence of sugar may 

 be readily demonstrated by means of the following simple 

 experiment: Take three test tubes containing the sap squeezed 

 from a plant. This sap always contains a certain amount of 

 protein. One of these tubes should be left as a check; the two 

 others should be frozen, after a little sugar has been added to 

 one of them. Upon thawing the contents of both tubes, it will 

 be seen that in the tube that received sugar the proteins are in 

 the same colloidal condition as in the control. The tube frozen 

 without the addition of sugar will show a flaky precipitation of 

 the coagulated protein (Fig. 113). 



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 greenhouses are easily killed by freezing, while 

 those grown in the open at low temperatures 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 can be hardened to withstand a mild 



