THE MOLECULAR FORCES IN PLANTS. 129 



tliese conditions freezes therefore not at C., like pure water, 

 bat at lower temperatures. The state of equilibrium between the 

 molecules of the water, and the molecules of the salt or those of 

 the solid bodies, is not disturbed, so that the formation of ice can 

 take place, till the temperature has fallen below C. Certainly 

 on formation of the ice a considerable quantity of heat is set 

 free ; but still the temperature of the salt solution, or of the 

 solids permeated by water, does not rise quite to 0C., since a 

 certain quantity of heat is employed in separating the mole- 

 cules of water from the molecules of salt, or from the molecules 

 of the solid bodies, as the case may be. 



1 On this subject and what follows see Miiller-Thurgau, Landwirthschaftl. 

 Jahrbiicher, Bd. 9, p. 133. 



50. Death resulting from Exposure to too High a Temperature. 



We first employ young plants of Zea, Nicotiana, Cucurbita, 

 Phaseolus, or Tropseolum, growing in small flower pots. The 

 plants are suitable for the experiments as soon as a few leaves 

 have unfolded. We then warm the air under the bell of a thermo- 

 stat to the temperature whose effect is to be studied, and when the 

 temperature has become constant at this level, one of the plants ia 

 introduced. We wait until the desired temperature is again 

 reached within the apparatus, and then leave the plant exposed 

 to it for a certain time. It is often desirable, in order to obtain 

 more rapid adjustment of temperature, to water the soil in the 

 pots with warm water. One thermometer is placed in the soil in 

 which the plant is rooted ; another is suspended within the belt- 

 glass so as to touch the aerial parts of the plant. We can now 

 vary the experiments in many ways. We may, e.g., leave our re- 

 search plants for half an hour in the apparatus with the air at 

 a temperature of 40 to 45 C., or we may expose them for 

 10-30 minutes to a temperature of 52 C. We then remove the 

 plants from the thermostat, expose them to normal conditions of 

 environment, and observe their further behaviour. Usually they 

 may be kept for half an hour in air at a temperature of 40 C. 

 without injury, but exposure for a period of from 10 minutes to 

 half an hour to a temperature of 52 C. as a rule kills them. It 

 must be observed, however, that the plants do not by any means 

 immediately die off after exposure to too high a temperature, e.g. 



PP. K 



