MOTION OF THE JUICES. 369 



and animals, may accufiiulate large supplies of all the 

 elements of the Volatile part of Plants. Carbon, cer- 

 tainly in the form of carbon dioxide, probably or possi- 

 bly in the condition of Humus (Vegetable Mold, Swamp 

 Muck), may thus be put as food, at the disposition 

 of the plant. Nitrogen is chiefly furnished to crops by 

 the soil. Nitrates are formed in the latter from Tarious 

 sources, and ammonia-salts, together with certain proxi- 

 mate animal principles, viz., urea, guanin, tyrosin, uric 

 acid and hippuric acid, likewise serve to supply nitrogen 

 to vegetation and are often ingredients of the best ma- 

 nures. ■ It is, too, from the soil that the crop gathers all 

 the Water it requires, which not only serves as the fluid 

 medium of its chemical and structural metamorphoses, 

 but likewise must be regarded as the material from which 

 it mostly appropriates the Hydrogen and Oxygen of its 

 solid components. 



§3- 



THE JUICES OF THE PLANT, THEIB NATURE AND 

 MOVEMENTS. 



Very erroneous notions have been entertained with 

 regard to the nature and motion of sap. It was formerly 

 taught that there are two regular and opposite currents 

 of saj^ circulating in the plant. It was stated that the 

 "crude sap" is taken up from the soil by the roots, 

 ascends through the vessels (ducts) of the wood, to the 

 leaves, there is concentrated by evaporation, "elabor- 

 ated" by the processes that go on in the foliage, and 

 thence descends through the vessels of the inner bark, 

 nourishing these tissues in its way down. The facts 

 from which this theory of the sap naturally arose admit 

 of a very different interpretation ; while numerous con- 

 24 



