ADJUST.MENT TO TEMPERATURE 97 



which are seriously injured or killed by temperatures several degrees 

 above the freezing-point. If allowed to become warm very gradually 

 after freezing, many plants remain turgid in place of wilting, and 

 thus survive. This result appears to be due to the gradual absorp- 

 tion of the melting crystals by the cytoplasm, and there is in con- 

 sequence no loss of turgidity. When the thawing takes place 

 rapidly, the protoplast is unable to do this, and the ice crystals 

 melt and fill the intercellular spaces. The cells become flaccid, 

 and the plant wilts and dies. In the same habitat, plants differ 

 much in their ability to resist frost, and this is also true of different 

 parts of the same plant. Aerial parts naturally freeze first, but 

 among these the newest and most active tissues, i.e., those con- 

 taining the most water, succumb first. Flowers are most easily 

 damaged, then the leaves, next the stems, and last of all the 

 roots, which are more or less protected by the soil. Upon high 

 mountains, where frosts may occur at almost any time during 

 the summer, plants become unusually resistant, and able to with- 

 stand repeated freezing and thawing. 



Experiment 32. Effects of freezing. Soak peas in water for twenty- 

 four hours. Expose ten soaked and ten dry peas for twenty-four hours 

 to temperatures below freezing. Place the two sets of peas in moist 

 chambers, and compare their power to germinate. At the same time 

 place several sunflowers of different ages out of doors, together with 

 plants of a more woody species. Note the effect upon each. Transfer 

 some plants quickly to a warm room, and others more gradually. Cut 

 a section of a frozen leaf or stem, and compare with a section made of a 

 part that has thawed out. 



119. The sum of temperatures. The greater number of species 

 and of individuals pass through their entire life cycle without 

 being exposed to maximum or minimum temperatures. Extremes 

 of temperature have little significance for them. Their effect is 

 confined to the plants that appear very early in the growing 

 period, and those that linger toward the close. The activity and 

 growth of any plant depend upon its receiving the requisite amount 

 of heat during the growing period. Although temperature has no 

 power to change the form and structure of plants, its influence 

 upon size is very great, owing to its control over growth. The 

 sum of the temperatures which act upon a plant is of the first 

 importance in determining its general appearance. The effect 



