142 Temperature 



Extreme Temperatures 



Minimum Temperatures. We may consider first the danger to the 

 organism of the freezing, or the actual congeahng, of its hving tissues. 

 Freezing is hkely to produce mechanical harm in the form of the rup- 

 ture of cell walls and the stoppage of circulation. When ice crystals 

 are formed, they also withdraw water from neighboring areas, pro- 

 ducing a condition similar to desiccation in the surrounding cells. No 

 tissue will freeze at 0°C. The concentration of solutes in body fluids 

 and tissue fluids causes a depression of the freezing point with the 

 result that even unprotected protoplasm will not freeze until a tem- 

 perature a few degrees below zero is reached. For this reason many 

 tissues can be active at 0°C; rye seeds, for example, will germinate on 

 ice. 



In situations where the temperature drops considerably below zero 

 the danger of freezing is ever present. Some forms have special 

 adaptations for lowering the freezing point of their tissues still farther 

 so that congealing does not take place until a very low temperature is 

 reached. In addition a few species are able to withstand actual freez- 

 ing for short periods of time. The green alga, Clilorella, while in 

 active vegetative condition, has been frozen at — 182°C for 1 hour 

 without harm. At subzero temperatures ice crystals are known to 

 occur in the needles and wood of the Norway spruce, and their pres- 

 ence evidently produces no serious injury to the tree. Among the 

 higher animals the Alaskan blackfish has the ability to recover normal 

 activity after being frozen for periods of 40 minutes at temperatures 

 down to — 20°C. 



Many plants and animals are killed by temperatures that are too low 

 for them but are nevertheless far above values at which tissues would 

 actually freeze. Similarly, at the other end of the scale we find many 

 instances of death from excessively high temperatures but at values 

 below those producing heat coagulation. Lethal extremes vary greatly 

 from species to species, and the organism as a whole may be killed by 

 a degree of chilling insufficient to cause direct damage to individual 

 protoplasmic structures. Temperatures that are too low for some 

 species may be favorable, or even too high, for others. The English 

 daisy grows and flowers best when subjected to night temperatures 

 below 10°C and dies if kept for long periods above 20°C. In contrast 

 the African violet is killed by long exposure to night temperatures 

 below 10°C and grows best and flowers when the thermometer stands 



