96 BIOLOGY OF PLANTS. 



transpired by the leaves, it is not all thus disposed of; the hy- 

 drogen, which is found in such abundance in vegetables, can 

 be obtained from no other source. The vital power is able 

 to decompose the water, and assimilate its hydrogen, while 

 its oxygen is either combined with some other body, and re- 

 jected as excretory matter ; or assimilated also to the vegeta- 

 ble structure. Many vegetable bodies, as woody fibre, con- 

 tain carbon and the elements of water, hydrogen and oxygen, 

 in the same relative proportions as in water, and hence water 

 may be directly assimilated. 



III. Water in the state of vapor ministers to the life and 

 growth of plants, in a manner but little less effectual than in 

 the liquid form. It boils and is converted into steam at 212° 

 F., but the quantity of vapor thus produced is very small, com- 

 pared with that which rises at all temperatures* by a process 

 called evaporation, a process which is promoted by a high 

 temperature, extent of surface, and the dryness and motion of 

 the atmosphere. 



1. By the constant evaporation of water from the surface 

 of the ocean and the land, from the leaves of vegetables and 

 the bodies of animals, large quantities of vapor are thrown off 

 into the atmosphere. This quantity is found to vary with the 

 temperature. At 50° F. the atmosphere contains about j\j 

 or yL part of its weight. At 100° F. about ■j\ or j\y part of 

 its weight. When the temperature is diminished, it is con- 

 densed and appears in the form of vapor, or is deposited in 

 the state of dew ; hence the diminution of temperature, dur- 

 ing the night, precipitates a quantity of moisture upon veg- 

 etables, and restores their freshness and vigor. 



2. The leaves and 7'oots have the power of absorbing the 

 water thus thrown upon them, and, according to Liebig, af- 

 ter their organs of nutrition are fully matured, derive nearly 



* Major Sabine states that in the intense cold of the polar seas, not 

 only living bodies, but the very snoto smokes, and fills the air with 

 vapor. 



