96 PLANT PHYSIOLOGY 



100° C. Similarly dry plant parts can endure very low 

 temperatures. Many seeds are not killed by an ex- 

 posure for several hours to the temperature of liquid 

 hydrogen (below — 250° C). The latter is also true for 

 many single-celled water plants that must contain plenty 

 of water, e.g. diatoms, bacteria, etc. On the other hand 

 many watery tissues are killed by a temperature that does 

 not reach the freezing point. Just the reason for this is 

 unknown. It is here suggested that at these low 

 temperatures certain processes continue which result in 

 the accumulation of poisons, while the processes that 

 would usually destroy these poisons, are prevented by the 

 low temperature so that in reahty the death of the plant 

 would be due to poisoning. 



145. Freezing of plants may cause death in several 

 ways: (1) the ice crystals formed may rupture the 

 cells or disrupt the tissues; (2) the water may escape 

 into the intercellular spaces and be frozen there and on 

 thawing rapidly may remain outside the cells filling up 

 the intercellular spaces and cutting off the air supply; 

 (3) the withdrawal of water from the protoplasm by freez- 

 ing may so increase the concentration of certain sub- 

 stances dissolved in the cell sap that the cells are killed. 

 Upon the whole subject considerable uncertainty rests. 



146. Effect of Poisons. Many substances are poison- 

 ous to living plant cells. The effects are almost as varied 

 as the types of poisons. Some, like the strong acids, 

 simply decompose the protoplasm and cell walls and so 

 destroy life; others, Hke the salts of the heavier metals, 

 coagulate the protoplasm; others even in minute quanti- 

 ties interfere with the nutrition of the cell in a manner 

 not understood, and kill it. Thus one part of copper in 

 ten million parts of water will kill certain algae and fungi. 

 Hydrocyanic acid acts apparently by preventing the 



