ADJUSTMENT TO TEMPERATURE 95 



comes more and more noticeable. Heat penetrates the soil slowly, 

 either on account of poor conductivity or because of the capacity 

 of the water content for latent heat. The temperature of the 

 soil decreases rapidly in the first few decimeters, and at the depth 

 of a meter often remains constant throughout the growing season. 

 The air is ordinarily warmer in the daytime than the soil, especially 

 on sunny days. It loses heat more easily, however, and after a 

 sudden change or at night the soil is for a time warmer than the 

 air. On account of the high specific heat of water, dry soils are 

 regularly warmer than wet ones. 



ii6. The influence of other factors. Clouds and winds are 

 among the most important factors that modify temperature. 

 During the daytime clouds decrease temperature by absorbing 

 the sun's rays, while at night the effect is exactly opposite, on 

 account of the hindrance they offer to radiation. Winds raise 

 temperatures when they blow from a warmer region, and lower 

 them when they come from a cooler one. Humidity acts after 

 the manner of clouds, a humid air increasing the amount of heat 

 absorbed. Soil temperatures are affected not only by the amount 

 of water present, but by the character of the soil. Sand and 

 gravel are more easily heated than clay and loam, and the air 

 above them is also warmer because they lose heat more rapidly 

 by radiation. The effect of exposure is closely connected with 

 slope. Slopes that face the south and west receive the most sun- 

 shine, and are regularly warmer than north and east slopes. Cover, 

 whether dead or alive, reduces day temperatures by screening out 

 the sun's rays, and increases night temperatures by retarding 

 radiation. 



117. Favorable and unfavorable temperatures. It is hardly 

 necessary to point out that the temperatures of a plant's own 

 habitat are ordinarily the most favorable to it. It is likewise 

 clear that the plant is subjected to a considerable range of tem- 

 perature during the growing period. The point within this range 

 at which the plant functions best is the optimum. This varies 

 much, the optimum temperature for the seedling being lower than 

 for the fruiting plant. It is never a mere point, but is a space of 

 several degrees at least. In fact, for plants in their own habitats 

 it is largely hypothetical. This is not true for the extremes of heat 

 and cold which a plant can withstand and live. These are the 

 maximum and minimum respectively, and are actual points beyond 



