214 General Botany 



Hardening of plants. " Hardening " is a term applied by 

 gardeners to the practice of rendering young plants immune to 

 drought and frost injuries. Seedhngs grown in hotbeds and 

 greenhouses in early spring, if set out directly into the open 

 ground, are very susceptible to drought and freezing tempera- 

 tures. If kept, however, in cold-frames for a few days at tem- 

 peratures several degrees above the freezing point, they increase 

 in hardiness, and if set out will withstand frost. 



Investigation shows that hardened plants differ from tender 

 plants in having (i) more water-imbibing substance in the cells 

 and (2) in having more soluble proteins. The former prevents 

 the withdrawal of too much water from the cells when freezing or 

 drought occurs. The latter prevents the precipitation of the 

 proteins when the cell sap becomes more concentrated by the 

 partial withdrawal of water. Some plants may be hardened by 

 subjecting them to drought before freezing weather, and the 

 changes in the cells are quite similar to those that occur when 

 hardening is brought about by the low temperature of cold- 

 frames. 



The hardiness of certain varieties of peach is due to the slowness 

 with which they take up water in early spring. The cell sap in 

 the buds is consequently very concentrated, and hght frosts are 

 not sufficient to freeze the water in the tissues. 



If cultivated perennials are kept rather dry in the autumn, 

 they are much less likely to be winter killed than if they are kept 

 wet and green up to the time of kiUing frosts. 



Plants differ greatly in their ability to synthesize the water- 

 holding substances that produce hardiness. Wheat, for example, 

 hardens readily and can withstand drought and extremely low 

 temperatures. Oats, on the contrary, seems to lack these water- 

 holding substances and is resistant neither to drought nor to low 

 temperature. 



Winds. Winds and air currents are of importance, as they 

 affect the rate of transpiration or modify the temperature. Pre- 



