ADJUSTMENT TO TEMPERATURE 91 



termed thermotropic. The opening and closing of flowers and 

 flower heads are due to temperature, and to some degree doubt- 

 less the drooping of leaves at night and the movements of some 

 fruits arise from the same cause. While all functions, absorption, 

 photosynthesis, etc., owe much to temperature, certain ones, 

 digestion, respiration, growth, and reproduction, are particularh' 

 dependent upon it. This is likewise true of the germination of 

 the seed, which combines the first three functions. In connec- 

 tion with germination, temperature has a profound effect upon 

 ecesis, or acclimatization, and consequently upon the develop- 

 ment of vegetatiom 



112. The measurement of temperature. With the aid of the 

 thermometer, the determination of temperature becomes one of 

 the simplest of tasks. Since temperature is an extremely vari- 

 able factor, it is often convenient to use automatic thermometers, 

 or thermographs. When thermographs are not available, their 

 lack may be partially met by maximum-minimum thermometers, 

 which are used to record the extreme temperatures during day 

 and night. In all ordinary work, however, the use of the simple 

 thermometer in connection with simultaneous readings gives sat- 

 isfactory results. The thermometers used should be standard or 

 standardized instruments reading accurately to one degree, or, 

 better, to one fifth of a degree. As these are both delicate and 

 expensive, they must be used with great care, particularly in the 

 field. In making readings of air temperatures, precautions are 

 necessary to expose the bulb to the full effect of wind or sun, 

 when these are present, and to keep it away from the hand or 

 person. Moreover, in taking readings in rapid succession, the 

 instrument must be left in each position until the mercury becomes 

 stationary. In some cases, as when the wind blows fitfully, the 

 mercury rises and falls constantly. In simultaneous readings 

 this fluctuation may be ignored, and the precise position at the 

 time of the signal is the one taken. Otherwise the mean of the 

 fluctuation is taken as the proper reading. Readings at the sur- 

 face of the soil are made in the same way. In all cases the bulb 

 must touch the surface and not lie separated from it by a stratum 

 of air. The temperatures are different for the soil surface and 

 for dead or living vegetation lying upon it. Consequently this 

 fact must be taken into account in comparative readings made 

 in different stations. 



