VI. RELATION OF PLANTS TO TEMPERATURE 



139. General Relations of Temperature to Protoplasm. Tem- 

 perature is one of the most widely interlocking factors con- 

 cerned in the activity of protoplasm. This is due chiefly to 

 the fact that the characteristic processes of living matter are 

 essentially chemical changes, in which the molecular activity of 

 the engaged elements as well as their physical properties are 

 affected directly by the temperature. All absorptive and excre- 

 tory functions, respiration, enzymatic action, synthetic processes, 

 turgidity, germination, growth, reproduction and adaptive move- 

 ments are directly dependent upon the temperature of the body 

 of the plant, which follows more or less closely the medium in 

 which it exists. Variations in temperature inside the limits of 

 tonicity induce reactions expressed by altered metabolism, or 

 movements, and continued existence in any given average allows 

 it to become a distinctive formative factor. In addition it is also 

 to be said that the radiant energy of heat waves forms a source of 

 energy to the plant. 



140. Tonicity to Temperature. A maximum and minimum may 

 be distinguished for most plants. Between these lies a temperature 

 at which the greatest activity is reached, constituting the opti- 

 mum. The optimum shows great variation among dissimilar or- 

 ganisms. It varies from 26.6 C. to 37.7 C. among the seed 

 plants, and from 20 C. to 70 C. in the lower forms inclusive of 

 bacteria. 



The maximum temperature of the higher plants ranges from 

 37 C. to 46 C. according to the species, individual, and stage of 

 development. Among the simpler forms the maximum may be 

 much higher, especially in bacteria and algae living in warm 

 springs and thermal waters. At or above the maximum, proto- 

 plasm passes into a state of immobility known as heat rigor, from 



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