30 PLANT PHYSIOLOGY AND ECOLOGY 



taken to secure it before the mercury begins to rise in consequence 

 of stopping the movement. For this reason it is desirable to shade 

 the psychrometer with the body when looking at it in the sunshine, 

 and to take pains not to breathe upon the bulbs or to bring them 

 too near the body. At the moment when the wet bullj registers 

 the lowest point, the dry bulb should also be read and the results 

 recorded. 



39. Use of humidity tables. To ascertain the relative humid- 

 ity, the difference between the wet- and dry-bulb figures is obtained. 

 This difference, together with the dry-bulb temperature, is referred 

 to the tables. A variation in temperature has less effect than a 

 variation in the difference. In consequence, the dry-bulb reading 

 is expressed in the nearest imit, and the difference is reckoned to 

 the nearest .5. Since the humidity varies with the air pressure, 

 it is necessary to use the table computed for the normal barometric 

 pressure of the place under consideration. Humidity tables are 

 usually computed for pressures of 30, 29, 27, 2.5, and 23 inches (76, 

 73.5, 68.5, 63.5, 58.5 cm.). For mountain regions over 2100 meters 

 (7000 feet) additional tables are desirable, but the table for 23 

 inches will meet all ordinary requirements, since the effect of 

 pressure is small within the usual range of growing-period tem- 

 peratures. 



Experiment 4. Measuring humidity. Use a cog ps3'chrometer to 

 determine the range of humidity at 8 a.m., 12 m., and 4 p.m. "Slake 

 readings in quick succession in the plant-house, and in sun and shade 

 out-of-doors, and find the relative humidity for each. 



40. Method of habitat study. A real knowledge of physical 

 factors, and of the habitats which are constituted by them, can be 

 obtained only by the use of factor instruments in the field. Such 

 knowledge is of the most fundamental importance in discovering 

 the causes which control the functions and structures of plants, and 

 their grouping into plant formations. All these objects can be 

 obtained by establishing a series of stations, and using the members 

 of the class to take simultaneous readings in them at different times 

 of the year. The ideal method is to begin such a series just before 

 the opening of spring, and to continue it at proper intervals through- 

 out the entire growing period. This is scarcely feasible in the great 

 majority of cases, and the most practicable method is to take a set 

 or two of readings in the fall, and the same number in winter. Then, 



