Killing of Plant Tissue by Low Temperature 157 



lower the temperature of all the plants together to a point that would 

 probably kill the most tender, and after removing these to lower it 

 further. The rate of fall would thus be the same for all down to the 

 temperature at which the most tender were removed. 



The thermometer used in the earlier years of freezing was a 

 pentane thermometer graduated to one-half degrees. The zero 

 point was far enough above the bulb so that when the thermometer 

 was inserted through a cork at the top of the jacket of the freezing 

 chamber, the bulb would be on the same level with the plants. Later, 

 special mercury thermometers graduated to low temperature were 

 used. These were standardized by the makers. However, new 

 thermometers were checked by those used with previous work, and 

 also, checked from time to time, with standard thermometers of the 

 Columbia Branch of the United States Weather Bureau. No effort was 

 made to read the thermometers to closer than one-half degrees. Plants 

 on being removed from the freezer were always examined to see if the 

 tissue were frozen stiff. In freezing buds and woody tissue that 

 killed at a temperature lower than -15, the apparatus shown in 

 figure 2 was used. The twigs or pieces of tissue and the thermometer 

 were fastened to the inner cylinder which was filled with cotton. 

 This was set in a cylinder enough larger to leave a surrounding space 

 of about three-fourths of an inch. The second cylinder was about 

 six inches taller than the inner one, and was set with a one and one- 

 half inch space between them. There was about two inches of space 

 between the walls of this third cylinder and those of the one in which 

 this was placed. The fourth cylinder was well insulated by being 

 packed in dry saw dust. Ice and salt were first packed loosely and 

 then firmly in the space between the fourth and third cylinder. In 

 this way the temperature of the twigs could be lowered generally to 

 -17 C. When it was necessary to secure a lower temperature, the 

 space between the third and second cylinders was packed loosely and 

 later firmly with salt and ice. In this way the temperature of the 

 air surrounding the inner cylinder could be gradually lowered at the 

 rate of two to three degrees an hour after the freezing point was 

 reached. By packing salt and ice to the top of the second cylinder 

 the temperature from top to bottom of the inner c>linder would 

 vary but little. However, the freezing tissue to be compared and the 

 thermometer bulb were always kept the same distance from the bot- 

 tom of the cylinder. Since these cylinders were of galvanized iron 

 and would conduct heat rapidly, it would seem probable that the tem- 

 perature aroimd the central cylinder would not vary. However, 

 fearing that there might be some such variation in temperature, the 



