IMPORTANCE OF WATER 



125 



ing. Some plants, such as the cucumber, tomato, dahlia, and potato, 

 are easily killed by even a mild frost. Others are extremely hardy, 

 as, for instance, winter rye and the conifers. Frost-killed plants 

 look as if scalded ; they lose their turgidity , and their leaves rapidly 

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

 potato tuber or the beet root thaw out after having been frozen, 

 water flows out of them as easily as from a sponge. 



This ready loss of sap by frost-killed organs was explained for 

 a long time as being due to the fact that the freezing water, in 

 expanding, ruptures the cell 

 walls. This supposition, how- 

 ever, proved false. Micro- 

 scopical 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. 47). 

 The cell walls in frost-killed 

 plants remain uninjured. The 

 cause of the death of these 

 plants must be sought, not in 

 the rupture of the cell walls, 

 but in the changes of the 

 protoplasmic membrane, 

 primarily its coagulation. This coagulation is the result of the 

 formation of ice in the intercellular spaces and the withdrawing 

 of water from the cells. The cell sap becomes increasingly more 

 concentrated, while the protoplasm is gradually deprived of water. 

 Besides, the protoplasm becomes exposed to the pressure of the 

 growing ice crystals. As a result, an irreversible coagulation of 

 the colloidal substances of the protoplasm takes place. After 

 thawing, it is dead and has lost its impermeability. 



Hence, frost killing must be regarded not as the direct influence 

 of cold on the protoplasm but as an indirect influence, the desicca- 

 tion of protoplasm due to the freezing out of water. One of 

 the most direct proofs of this statement is the fact that in the state 

 of undercooling, without ice formation, the plant is able to 

 endure such low temperatures as would kill it if freezing had 

 occurred. 



Fig. 47. — Accumulation of ice in the 

 intercellular spaces of the frozen stem of 



Fritillaria (after Sachs). 



