- 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. 
Thermometers 
94. Airthermometers. 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 
