70 



THE HABITAT 



100. Place and height. For general air temperatures, thermograph and 

 thermometer readings are made at a height of 3 feet (i meter). Soil tem- 

 peratures are regularly taken at the surface and at a depth of i foot. 

 When a complete series of simultaneous readings is made in one station, 

 the levels are 6 feet and 3 feet in the air, the surface of the soil, and 5, 

 10, and 15 inches in the soil. When sim and shade occur side by side in 

 the same formation, as is true of many thickets and forests, surface read- 

 ings are regularly made in both. Similarly, valuable results are obtained 

 by making simultaneous readings on the bare soil, on dead cover, and upon 

 a leaf, while the influence of cover is readily ascertained by readings upon 

 it and beneath it. A full series of station readings made at the same time 

 upon north, east, south, and west slopes is of great importance in studying 

 the efifects of exposure. 



Expression of Results 



101. Temperature records. Neither field nor permanent form is re- 

 tjuired for thermographic records, other than the record sheet itself, which 

 contains all the necessary information in a fairly convenient form. Al- 

 though the temperature of a particular hour and day can not be read at a 

 mere glance, it can be obtained so easily that it is a waste of time to make 

 a tabular copy of each record sheet. For thermometer readings, either sin- 

 gle or in series, 'the following form is used : 



102. Temperature sums and means. The amount of heat, i. e., the num- 

 ber' of calories received within a given time by a definite area of plant sur- 

 face, can be determined by means of a calorimeter. From this the tempera- 

 ture sum of a particular period may be obtained by simple addition. In 

 the present condition of our knowledge, it is impossible to establish any 

 exact connection between such results and the functional or growth effect 

 that can be traced directly to heat. As a consequence, temperature sums 

 do not at present contribute anything of value to an understanding of the 

 relation between cause and effect. The mean daily temperature is readily 



