76 EXPERIMENTAL PLANT PHYSIOLOGY. 



damage to the organism ; 3d, optimum temperature, in which 

 normal development proceeds ; 4th, maximum temperature, 

 at which the protoplasmic activity comes to a standstill without 

 necessarily injuring the organism ; 5th, destructive heat, pro- 

 ducing death by disintegration of the protoplasm. These divi- 

 sions vary greatly with each species. 



58. Sources of Heat. — The temperature of any plant is the 

 result of the heat it receives from several sources. A portion 

 comes directly from the heat-rays of the sunlight, as well 

 as from the light-rays which it is able to convert into heat by 

 means of chlorophyll, anthocyanin, and other coloring matters. 

 Another portion is received from the soil, which is generally 

 more constant in temperature than the air. According to 

 Kerner the soil of a mountain at a height of 2200 meters is 

 3.6° C. higher than the surrounding air. Another and by no 

 means unimportant source of heat is the combustion of the 

 carbon compounds in the plant. (See Experiment 53.) 



59. Influence of Temperature on Geographical Distribution. — In 



consequence of the obliquity of the ecliptic, no place on the 

 earth has the same temperature during the entire year, disre- 

 garding even the changes of day and night. Fluctuations in 

 temperature vary greatly with the locality : it is greater in 

 valleys and at the poles than it is on mountains and at the 

 equator. Fluctuation further depends upon the continental or 

 oceanic, position of a place. Again, between the elevated cold 

 regions of the warmer zones and the polar regions there is the 

 difference of short period of daylight and the long summer on 

 one hand and the longest period of daylight, and a short 

 summer on the other. These conditions of temperature, to- 

 gether with those of rainfall and soil, are the most important 

 factors in the geographical distribution of plants. The regions 

 which are not subject to extremes of temperature will be found 



