442 



BOTANICAL GAZETTE 



MAY 



determined, measurements to o . i°C. or less were found to be sufficiently 

 accurate for this work. 



Fig. I shows the arrangement for temperature measurement as well 

 as the means for securing undercooling of the tissues. Directions for 



constructing the thermal junctions 

 are to be found in the pubUcations 

 of White from the Carnegie Insti- 

 tution of Washington.^ The con- 

 stant temperature junction of the 

 thermocouple was placed in a thin- 

 walled tube filled with oil and 

 immersed in a slush of clean snow 

 in distilled water contained in a 

 Dewar beaker. It was found that 

 the temperature of this junction 

 could be kept constant for a long 

 time within a few thousandths of 

 a degree Centigrade, as shown by a 

 standard Beckmann thermometer. 

 In calibrating the couple, the 

 junction to be placed within the 

 leaf was placed in a Dewar beaker 

 similar to the former arrangement, 

 with solutions having freezing 

 points a few degrees below zero. 



A number of tissues were tested 

 with this apparatus, including car- 

 nation stem, cabbage \ea.i,Echeveria 

 leaves. Cineraria petiole, tomato 

 petiole, and others. In general the 

 results show that plants with heavy 

 epidermal coverings of wax or 

 Fig. I trichomes can undergo a much 



greater undercooling than such 

 plants as tomatoes, in which the epidermal walls are thin and non- 

 resistant. 



* White, W. P., The thermoelement as a precision thermometer. Physical 

 Review 31:135. 1910. 



, The constancy of thermoelements. Physical Review 23:449. 1906. 



White, W. P., Dickinson, H. C, and Mueller, E. I. The calibration of 

 copper-constantan thermoelements. Physical Review 31 : 159. 1910. 



