RESISTANCE OF PLANTS 



351 



ground level or below it. Their ability to overwinter then must 

 depend upon their hardening capacity. 



An explanation of frost resistance is to be found in studies of 

 the phenomena that take place in the cells and tissues of plants 

 during freezing. Plants killed by frost look as if they had been 

 scalded. They lose their turgidity, and their leaves rapidly turn 

 brown and become dry. When such fleshy tissues as the potato 



/ 



i 



^M 





^-i. 



Fig. 112. — Ice masses in frozen carrot. Note arrangement of ice and breaking 



apart of tissues {after Harvey). 



or the beet thaw out after having been frozen, water flows out 

 of them as easily as from a sponge. 



For a long time, this ready loss of sap by frost-killed organs 

 was explained as being due to the fact that the freezing water, 

 in expanding, ruptures the cell walls. This supposition, how- 

 ever, proved false. Microscopical observations have shown that 

 water contained in the cell walls is frozen first and that ice is 

 formed not within the cell but in the intercellular spaces (Fig. 

 112). The cell walls in frost-killed plants remain uninjured. 



