78 EXPERIMENTAL PLANT PHYSIOLOGY. 



EXPERIMENT 98. 



FREEZING OF A SALT SOLUTION. 



Partially freeze a solution of potassium bichromate or copper 

 sulphate. The frozen portions are distinguished from the concen- 

 trated fluid by the paler color. The freezing begins at a tempera- 

 ture a few degrees below zero C. 



EXPERIMENT 99. 



^FREEZING OF A BEET. 



Place a section of a Beet, a centimeter thick, well washed and 

 dried, in a dish covered with a glass plate to prevent evapora- 

 tion, at a temperature of 6 degrees below zero C. When the 

 section is frozen, the surface will be covered with a layer of ice,, 

 which when examined with the microscope, at a temperature below 

 zero, will be found to consist of parallel crystals. A very heavy ice- 

 layer is found on the under side of the section, where it has been 

 in contact with the dish. The ice is not colored red, proving that 

 not cell-sap but pure water drawn from the cell has been frozen. 

 No rupture of the cell-wall occurs in the freezing of living cells, as 

 would be the case if the enclosed fluid were frozen. 



EXPERIMENT 100. 



FREEZING OF SPIROGYRA. 



Freeze some Spirogyra filaments in a drop of water on a glass 

 slide. After thawing no rupture of the cell-walls appears. 



EXPERIMENT 101. 



FREEZING OF POTATOES. 



Place some Potatoes in a temperature of 5 to 10 degrees below 

 zero centigrade, over night. They are frozen hard, and upon thawing 

 become very soft, allowing the sap to be forced out by the lightest 

 pressure. Their power of germination is lost, and they easily rot. 

 Whether the Potatoes are thawed quickly or slowly is a matter of 

 indifference. 



61. Relation of Moisture to Freezing. Low and high tern- 

 peratures are destructive to plants and plant-organs in propor- 

 tion to their richness in water. 



