FROZEN PLANT-ORGANS. 493 



The death by freezing of plants, or certain parts of plants, 

 is usually due to a rapid thaw rather than to the direct effects 

 of the low temperature to which they have been exposed. 

 This is because, owing to the low temperature, the liquid 

 contents of the affected tissues becomes denser, and a change 

 ensues in them. The cell-sap, when converted into ice, 

 expands 10 per cent., and sets free part of the air which it 

 contains ; this increases the size of the pores and the per- 

 meability of the membranous lining of the cell-wall, which 

 loses its powers of resistance to the passage through it of 

 certain substances, and allows the cell-sap to pass into the 

 intercellular spaces of the plant, where it freezes. The 

 injured tissues thus become limp from loss of water. 



A similar result happens in the case of frozen starch-paste, 

 in which tlie water and starch become separated, and will 

 not reunite after a thaw. The air escaping from the frozen 

 tissues may also decompose the chlorophyll, and hence the 

 brown and eventually black colour of the dead organs. If, 

 however, the ice formed in the intercellular spaces thaws 

 slowly, the cell-wall may recover its normal elasticity and 

 reabsorb the water before the chlorophyll has decomposed. 

 With a rapid thaw this is impossible, as the water then remains 

 in the intercellular spaces, and death ensues. 



The effect of allowing the thawed water to become 

 reabsorbed may be well observed in a meadow after a sharp 

 frost in May, when the grass has been in full growth. No 

 bad results follow from the frost, unless men or animals tread 

 on the frozen grass, but wherever they do, the crushed grass 

 appears black and dead, as if singed by a red-hot iron. This 

 is because the crushed tissues will not allow the return of the 

 sap when the thaw sets in. The more water an organ or plant 

 contains, the more it is subject to be frozen. The old theory 

 that plant-cells are split by the freezing of the cell-sap, and 

 consequent expansion of the ice, is not true; in the first 

 place, the cell is not filled with sap, and secondly, the cell- 

 wall is sufficiently expansible to resist an extension of j^th 

 of its volume, supposing it were full of sap and the sap 

 converted into ice. 



Hartig states that cortex and bast containing concentrated 



