THE MOLECULAR FORCES IN PLANTS. 131 



immersed in water at 75 C., the leaves almost immediately be- 

 come discoloured, and are then dead. Water at 55 C. kills the 

 leaves within two minutes. 



To demonstrate the important fact that plant structures, es- 

 pecially seeds, bear exposure to high temperature far better when 

 dry than when saturated with water, we use the apparatus de- 

 picted in Fig. 44. The beaker B is filled with water. Through 

 the large cork with which it is closed pass the thermometer T, and 

 the two test-tubes P, P'. These last are fitted with corks, through 

 which also pass thermometers. The whole arrangement is sup- 

 ported on a ring stand, and dips into the water of a water-bath. 

 We now heat with a gas or spirit flame, till the thermometers in 

 the test-tubes indicate the temperature at which we wish to ex- 

 periment (e.g. 50 C., 60 C., or 70 C.). Air-dry seeds (50-100) 

 are now placed in one of the test-tubes, soaked seeds in the other. 

 We use seeds of Pisum, Zea, or Triticum, and leave them in the 

 apparatus for some time (say an hour), keeping the temperature 

 constant. The seeds are then laid in sawdust, and exposed to 

 normal conditions for germination. The air-dry seeds still prove 

 in part capable of germinating ; the soaked seeds all perish, and 

 do not germinate. 



Air-dry grains of Pisum, Zea, or Triticum may be exposed for 

 an hour to temperatures of 65 C. or 70 C., without all of them 

 losing their power of germinating, but on the other hand, their 

 ability to germinate is certainly more or less diminished. I ex- 

 posed air-dry grains of wheat for an hour to a temperature of 

 62 C., and a fairly large percentage of them were found to be 

 still capable of germination, but soaked seeds similarly exposed 

 for an hour to a temperature of 62 C. all perished. 1 



1 Literature : Sachs, Flora, 1864, p. 5, and Handbuch d. Experimentalphy- 

 siologie de.r Pfianzm, 1855, p. 64. Further see Detmer, Vcrgleichende Physio- 

 logic d. Keimunysprocesses d. Samen, 1880, p. 401 ; Hohnel in Fr. Haberlandt's 

 Untersuchungen auf dem Gebiete des Pflanzenbaues, Bd. 2, p. 77, and Detmer, 

 liotan. Zcitunrj, 1886, No. 30. 



51. Changes experienced by Plants in Death caused by Exposure 

 to too High Temperatures. 



We take a young bud leaf of Elodea canadensis, and after 

 ascertaining that the protoplasm of its cells is in active movement, 

 immerse it for a short time (say one minute) in water at a tern- 



