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, ete. On the other hand 
many watery tissues are killed by atemperature 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 reality 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, like 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 
