92 PLANT PHYSIOLOGY AND ECOLOGY 



113. Soil temperatures are determined by means of the instru- 

 ments used in the air, or by means of thermometers specially 

 constructed for the purpose. The latter are thermometers pos- 

 sessing a very long tube, the whole instrument being encased in a 

 wooden jacket for protection. The soil thermometer is placed 

 in the ground with the bulb at the desired depth. The scale is 

 then above the surf.ace, where it may be read directly. Such 

 thermometers, however, are relatively expensive. Their placing 

 requires considerable time and trouble, and their use is restricted 

 to the few places where they would be entirely free from dis- 

 turbance. Soil temperatures are relatively constant, especially 

 at depths greater than three or four decimeters. In consequence, 

 frequent readings of them are unnecessary, and the simplest 

 thermometer yields satisfactory results. Soil temperatures are 

 usually measured in connection with water content, the hole 

 from which the soil sample is taken serving for the temperature 

 reading. Unless the latter is made as soon as the sample is re- 

 moved, the hole is stopped with a cork to prevent any change 

 in temperature. In making the reading, the thermometer is 

 lowered carefully into the hole, the cork is put in place, and the 

 instrument allowed to remain for several minutes to make sure 

 that the mercury indicates the proper temperature. In the case 

 of holes deeper than the thermometer is long, a cord is tied to 

 the top of the latter to assist in raising or lowering it. In obtain- 

 ing the temperature, the thermometer is not lifted out of the 

 hole, but is raised in the latter until the upper end of the column 

 can be read. With a little practice, this can be done very easily 

 before the column begins to rise or fall. When several soil read- 

 ings are made in one station on the same day, the use of the cork 

 as a stopper makes it possible to use the same hole without appre- 

 ciable error. If readings are made on different days, it is desir- 

 able to bore a new hole. 



114. Plant temperatures. Unlike the warm-blooded animals, 

 plants do not possess temperatures that are independent of the 

 surrounding medium. Certain plant activities, notably respira- 

 tion, bring about an evolution of heat, but in the vast majority 

 of cases this is so slight as to have no appreciable effect upon the 

 plant. As a consequence, plants and plant parts tend to assume 

 the temperature of the medium, viz., air, soil, or water, in which 

 the}^ are found. In other words, they behave essentially as non- 



