270 COMPENDIUM OF GENERAL BOTANY. 



Effects of Extreme Temperatures. Seeds of plants in a dry 

 state, may withstand very low temperatures. The same is true 

 of spores, yeast-cells, and the Schizomycetes. Well-dried seeds 

 may resist a temperature of 120 C. without losing the power 

 of germination. " Sterilization " in bacteriology requires that 

 liquids should be boiled for hours in order to kill all the germs 

 of fungi, or exposed to dry heat at 130 to 140 C. for a longer 

 period. 



In general it may be stated that plants and parts of plants 

 with a low percentage of water will withstand the extremes 

 of temperature better than succulent plants. Lichens, for 

 example, will resist the extremest cold of winter. Winter wheat, 

 without being covered by snow, will resist a temperature of 

 10 C., or even lower ; Coleus is killed by a temperature of 1 to 

 1.5 C. Succulent plant-portions of various phanerogams are 

 killed by a temperature of 45 to 50 C. 



Freezing does not always kill the plant ; death sometimes re- 

 sults during the process of thawing. If this process is allowed 

 to proceed slowly the life of the plant may continue. Freezing 

 inhibits turgescence, that is the water within the cells escapes 

 into the intercellular spaces. As a rule, ice-formation begins 

 outside of the cells, in the intercellular spaces, when the temper- 

 ature sinks slowly to 5 C., or even lower. A sudden and exces- 

 sive fall in the temperature causes rupturing of the cells and 

 tissues, due to the pressure of the rapidly forming ice-crystals. 

 When the process of thawing is sufficiently slow the water is 

 again taken up by the remaining living cells. However, accord- 

 ing to FRANK and MULLER-THURGAU, death may result during the 

 process of freezing. 



B. EFFECT OF LIGHT. 



(a) Production of Light. 



Many plants and parts of plants are luminous in the dark ; 

 for example, certain fungi (bacteria). Since this phenomenon 

 ceases on the exclusion of O, it is in all probability a process of 

 oxidation. 



