THE METABOLISM OF PLANTS. 2/3 



The death of cells which contain much water when ex- 

 posed to a very low temperature, appears to depend upon 

 the conversion of the water into ice. The effect of freezing 

 is, as Sachs has shewn, that the parenchymatous tissues 

 become ruptured, and that on the surface of the isolated 

 masses of tissue radially arranged prisms of ice are formed. 

 The effect upon each individual cell is then this, that 

 the water which is present in it, either as cell-sap or as 

 saturating the protoplasm and the cell-wall, is gradually 

 attracted to the surface. This process necessarily involves 

 a considerable disturbance of what we may term the equili- 

 brium of the cell, in that the cell-sap becomes diminished in 

 quantity, and that what of it remains is very concentrated, 

 that is, that it holds in solution a relatively larger quantity 

 of the substances present in it than is normally the case. 

 Further, it appears from the observations of Kunisch that 

 the lowering of the temperature of the cell-sap may be 

 accompanied by chemical changes in the substances which 

 it holds in solution, and it is possible that these changes 

 may be prejudicial. There can be little doubt that the 

 leathery brown colour which is assumed after exposure to 

 frost by the persistent leaves of certain plants is due to 

 changes of this kind (see p. 267). Finally, if the exposure 

 be long continued, it will lead to the actual disorganisation 

 of the protoplasm, and therefore also to the death of 

 the cell. 



Provided that the protoplasm has not undergone dis- 

 organisation, the formation of ice does not necessarily involve 

 the death of the cell. If a frozen organ be thawed slowly, 

 so that its cells can gradually absorb the water which they 

 have lost, equilibrium will be restored in the cells and they 

 may continue to live. If, on the contrary, the thawing be 

 rapid, so that the water, instead of being absorbed by the 

 cells, escapes into the intercellular spaces, then the organ 

 is killed. 



Sachs found, for instance, that leaves of the Beet and of the Cabbage 

 frozen at from - 4 C. to - 6 C. and thawed either in air at 2 3 C. or in 

 water at 6 10 C. were killed, whereas, when they were slowly thawed in 



V. IS 



