THE PLANT IN ACTION. 249 



are changed from the mineral to the organic state by 

 the plant itself. 



The root, then, is an important organ of absorp- 

 tion. Its cellular extremities are very permeable, 

 and the water of the soil tends to penetrate them. 

 By the various agencies just explained, it rises, 

 through the cells and ducts, to the top of the plant, 

 and escapes into the air by way of the leaves. 

 Plants are said to absorb carbonic acid, ammonia, 

 and sometimes vapor of water, directly from the air 

 by their leaves, but the point is not well established. 



EXERCISE LXXVI. 

 Evaporation and Digestion. 



When the water of rains and dews, with the ma- 

 terials it has dissolved from air and earth, enters the 

 plant, it takes the name of ascending sap. It thick- 

 ens a little as it rises, by dissolving substances con- 

 tained in the cells, and, on reaching the leaves, it 

 undergoes various changes, and a large portion of its 

 water escapes into the air by evaporation. The ra- 

 pidity of its exhalation depends upon sunshine, the 

 warmth and dryness of the air, and the structure of 

 the leaves. A sunflower, with five thousand six hun- 

 dred and sixteen inches of leaf-surface, was found, 

 by experiment, to exhale from twenty to thirty ounces 

 in a day, while it lost only three ounces in a warm, 

 dry night, and none at all on a dewy night. A vine 

 with twelve square feet of evaporating surface ex- 



