32 



PLANT PHYSIOLOGY AND ECOLOGY 



bly near the middle of the series, and gives the signals by a shout 

 or whistle at the proper interval. Considerable care and prac- 

 tice are required in order to do the last satisfactorily. Suffi- 

 cient time must be given for the operation of the instrument 

 and the making of the record. In addition, a period which is 

 long enough for each instrument to reach the proper reading 

 must be permitted to elapse. For example, in a series which 

 contains a gravel slide and a forest, a thermometer which has 



Fig. 9. Observers making simultaneous readings of humidity in a series 

 of stations in the prairie formation at Lincohi. 



just been used for an air reading will require four or five times 

 as long an interval to respond to the temperature of the gravel 

 as to that of the cool forest floor. In such a series, the place 

 where the response is slowest or greatest often makes the best 

 signal-station. The instructor records the exact time of each 

 signal, and notes any general changes of sky or wind that pro- 

 duce fluctuations at the time of reading. Temperature, humidity, 

 and wind are read usually at Ih meters, and at the surface of 

 the soil. Soil temperatures are obtained from the holes left in 

 making soil samples. These holes are closed ^\ith corks to pre- 



