64 THE HABITAT 



daily duration, based upon full sunlight, is also of especial importance for 

 plants, and stations which receive both sun and shade during the day. 



TEMPERATURE 



93. In consequence of its indirect action, temperature does not have a 

 striking effect upon the form and structure of the plant, as is the case 

 with water and light. Notwithstanding, it is a factor of fundamental im- 

 portance. This is especially evident in the character and distribution of 

 vegetation. It is also seen in the germination and growth of plants, in the 

 length of season, and in the important influence of temperature upon hu- 

 midity, and hence upon water-content. Because of its intimate relation with 

 the comfort of mankind, the determination of temperature values has re- 

 ceived more attention than that of any other factor, and excellent simple 

 and recording instruments are numerous. For plants, it is also necessary 

 to employ instruments for measuring soil temperatures. The latter un- 

 questionably have much less meaning for the plant than the temperatures 

 of the air, but they have a direct influence upon the imbibition of water, 

 and upon germination. , 



Thenno7ncters 



94. Air thermometers. The accurate measurement of temperature re- 

 quires standard thermometers. Reasonably accurate instruments may be 

 standardized by determining their error, but they are extremely unsatis- 

 factory in practice, since they result in a serious waste of time. Accurate 

 thermometers which read to the degree are entirely serviceable as a rule, 

 but instruments which read to a fraction of a degree are often very much 

 to be desired. The writer has found the "cylindrical bulb thermometer, 

 Centigrade scale" of H. J. Green, to be an exceedingly satisfactory instru- 

 ment. The best numbers for general use are 247 and 251, which read from 

 -15 to 50 C. and are graduated in .2'". They are respectively 9 and 12 

 inches long, and cost $2.75 and $3.50. These instruments are delicate and 

 require careful handling, but even in class work this has proved to be an 

 advantage rather than otherwise. In making readings of air temperatures 



. with such thermometers, constant precautions must be taken to expose the 

 bulb directly to the wind and to keep it away from the hand and person. 



95. Soil thermometers. The thermometer described above has been used 

 extensively for soil temperatures. The determination of the latter is con- 

 veniently, combined with the taking of soil samples, by using the hole for 

 a temperature reading. When carefully covered, these holes can be used 

 from day to day throughout the season without appreciable error, even in 



