THE MOLECULAR FORCES IN PLANTS. J 25 



48. The Changes Which Take Place in Plants on Freezing. 



When plant structures are frozen, there is by no means, as ex- 

 perience teaches, any rupture of the cell-membrane. If, e.g., we 

 allow filaments of Spirogyra to freeze in a drop of water on a slide, 

 no breaks are to be perceived in the cell-walls after thawing. We 

 know further indeed that in the freezing of tissues, the formation 

 of ice takes place as a rule only in the intercellular spaces, etc., 

 and not in the cells themselves. 



Death from freezing is certainly to be referred to a breaking 

 down of the molecular structure of the protoplasm, as immediately 

 follows from the fact that, when the cells have been killed by 

 freezing, the protoplasm has lost its normal impermeability to 

 colouring matters, acids, etc. 



If we lay frozen pieces of beetroot in water at the ordinary 

 temperature, it takes up the red colouring matter in large quan- 

 tities, whereas the sap does not escape from the cells of unfrozen 

 pieces of beetroot when laid in water after being rinsed. Frozen 

 potato tubers after being thawed give up large quantities of sap 

 under slight pressure. The cells, owing to disorganisation of the 

 protoplasm, have lost their turgor, like those of frozen leaves, 

 which hang down limp, and rapidly dry up, as is very well seen 

 in frozen leaves of Begonia or Escheveria. 



If starch paste is frozen, and then thawed, we no longer have 

 before us a homogeneous fluid, but a spongy mass, the pores of 

 which are filled with fluid. Here obviously a rearrangement of 

 the molecules has taken place, and the experiment serves to illus- 

 trate to us in some measure some of the processes which take 

 place in the protoplasm of plant cells in freezing. 



49. Formation of Ice in Freezing Plants. 



A slice of beetroot, a few centimetres thick, well washed and 

 then wiped dry, is placed in a dish, which is covered with a glass 

 plate to prevent evaporation of water, and exposed to a tempera- 

 ture of say 6 C. When the piece of root is completely frozen, we 

 find that its surface is covered with a crust of ice, which, if 

 properly examined under the microscope at a temperature below 

 C., is seen to consist of rods of ice arranged parallel to one 

 another. The ice is particularly abundant on the under side of 

 the slice of root, i.e. where it has been in contact with the bottom 

 of the dish. This ice is not coloured red, from which it is clear 



