94 PLANT PHYSIOLOGY AND ECOLOGY 



in the evening, and at night. Compare and explain the temperatures 

 obtained. 



115. Variations of temperature. As in the case of light, there 

 is a daily and an annual fluctuation in temperature. The amount 

 of heat received depends upon the angle of the sun's rays and 

 their consequent absorption. The actual temperatures at the 

 surface of the earth are greatly modified by radiation, conduction, 

 and convection. In consequence the maximum daily temperature 

 does not occur at noon " sun-time," as in the case of light, but 

 several hours later, usually about 4 p. m. The minimum is not 

 reached at nightfall, but just before sunrise upon the following 

 morning. The maximum temperatures for the year do not occur 

 at the June solstice, but a month or two later. Similarly, the 

 minimum falls a month or more after the December solstice. 



The variation of temperature with latitude and altitude is 

 well known. Northern latitudes receive the sun's rays at a greater 

 angle than southern ones, and the absorption of heat by the atmos- 

 phere is correspondingly greater. In so far as absorption is con- 

 cerned, high mountains receive more heat than lowlands. The 

 loss by radiation, however, is so much greater that mountain 

 regions are uniformly colder than plains or lowlands lying on the 

 same parallel. This is due to the extreme rarity of the air, which 

 allows heat to pass through it readily. Although the air on moun- 

 tain tops is colder than that of the plains, the surface temperature 

 of the soil is often considerably higher. This difference, however, 

 is far overbalanced by the rapid radiation at night. Temperature 

 also varies with the slope. This is due to the fact that a square 

 decimeter of sunshine covers this amount of surface only when the 

 rays strike the latter at right angles. As the angle diminishes, the 

 rays are spread over more and more surface until at 10 a square 

 decimeter receives but 17% as much heat as at 90. It has already 

 been indicated that this has slight importance, owing to the fact 

 that stems and leaves have the same position upon a slope that 

 they do upon the level. Furthermore, temperature differs at 

 various levels in the air and the soil. Air and soil temperatures 

 naturally affect each other. The highest temperatures are usually 

 found between the two, i.e., at the surface of the soil. In both 

 directions the temperature rapidly decreases. In the air this 

 is due to the fact that radiation becomes imperceptible a short 

 distance above the ground, while the influence of the wind be- 



