GROWTH. 77 



most suitable for the greater number of species. While some 

 species of plants thrive with a low summer heat if the tempera- 

 ture does not sink to the destructive point in winter, others 

 endure a low temperature in winter very well if the tempera- 

 ture ascends high enough in summer to permit normal fruit- 

 formation. 



60. Freezing of Plants. Formerly it was believed that the 

 cell-sap was frozen by cold, that by the resultant expansion 

 the cell-walls were torn, and in this way the plant was killed. 

 It has, however, been demonstrated that a mechanical destruc- 

 tion of the cell by rupture does not take place, for the ice- 

 formation goes on only in the intercellular spaces, or, in the 

 simpler plants, in the water thrown out around the plant. It 

 is therefore now held that death by cold is the result of a 

 chemical process, which can occur at a temperature even above 

 freezing-point. 



Rapid or slow thawing of frozen plants has no influence 

 upon the life-energy of the plants. If, however, frozen plants 

 which are not killed are thawed slowly, the cells can reabsorb 

 the water from the melting ice-crystals around them and regain 

 their former turgor. If thawed rapidly, a portion of the water 

 of the ice-crystals is evaporated or driven away, and the cells 

 cannot regain their turgor. When a plant remains frozen for 

 some time, the water slowly evaporates from the crystals and 

 the plant is eventually dried. Therefore frozen plants may be 

 killed by loss of water, either through continued cold or rapid 

 thawing. 



Salt solutions freeze at lower temperatures than pure water, 

 and in their freezing the water is separated out in the form 

 of crystals. Cell-sap, a solution of several stable substances in 

 water, acts similarly, and plants may therefore endure a tempera- 

 ture many degrees below freezing-point without being frozen. 



