INFLUENCE OF CALORIC. 103 



more rapidly will the water evaporate ; thus the influence of 

 external temperature is modified, and the vital powers pre- 

 served. 



The quantity* of water thus transpired amounts, in some 

 cases, to the weight of the plant in twenty-four hours. 



The outer or upper surface of the leaf is further protected, 

 both from the too excessive heat of the sun's rays and from 

 the agency of water, by a thin lining of silica (flint), which 

 reflects the rays. This is particularly the case with herba- 

 ceous plants or grasses ; the epidermis or outer coat of the 

 stalks and leaves being composed in part of this substance. 



The temperature of the atmosphere is kept cool in the 

 spring, by the heat which is required to convert the snow and 

 ice into water, and the water into vapor. The process of 

 evaporation moderates the temperature during the summer 

 months, and the condensation of vapor in the autumn gives 

 out heat to the air, while its conversion into ice and snow 

 still further moderates the approach of winter. 



In this way, the vegetable kingdom is preserved from the 

 extremes of heat and cold, and from those sudden transitions 

 which might otherwise injure or destroy it. See page 35. 



4. During the winter, the roots of vegetables, of grasses 

 and trees are preserved from excessive cold, by the non-con- 

 ducting properties of snow and ice. The surface being fro- 

 zen, the heat of the earth is retained, and the principle of vi- 

 tality guarded from injury. The conversion of the juices of 

 the plant into ice does not take place until a low temperature 

 is reached. When this happens it developes its latent caloric, 

 and the temperature of the tree from this source must be, up- 

 on' an average, above that of the surrounding atmosphere. 



By the variations of temperature, winds are produced, and 

 the moisture precipitated to the earth in the form of dew and 

 rain. By these golden showers the processes of vegetation 

 are carried forward and enables the earth to yield her increase. 



* See Dana's Muck Manual, p. 225. 



