Chapter Six 

 ABSORPTION OF WATER AND MINERAL SALTS 



Animals, including man, depend on food made entirely 

 by plants, and made, as all the foods found on the earth are, 

 from simple materials, which animals cannot utilize as food. 

 From water and carbon dioxide plants make the world's sup- 

 ply of starch as is explained in Chapter 12, and the mineral 

 salts they use in the manufacture of protein. Plants can get 

 these materials only in the smallest units in which they exist, 

 called molecules or in at least some cases, atoms or ions. A 

 molecule is so small we can imagine nothing small enough 

 to compare with it. Bacteria, visible only with very good 

 microscopes, are very large in comparison. It would be 

 necessary to have at least a billion water molecules in a group 

 to be visible to the naked eye. In addition to being very 

 small, every molecule is in motion independent of the motion 

 of every other molecule. Food material for plants must 

 always be in that condition when it is absorbed by a plant. 

 Solids, as salts, or gases, as carbon dioxide, must be in the 

 form of molecules dissolved in water when they enter a cell 

 of a plant. 



Diffusion is the term applied to this movement of mole- 

 cules and ions when they move from regions of higher con- 

 centration toward regions of lower concentration. Each unit 

 moves independently of all other units which may surround 

 it, as for example, sugar molecules may move from a lump 

 of sugar into water while the water molecules move among 

 the sugar molecules. This movement results in a uniform 

 distribution of the sugar and water molecules. Other com- 

 mon illustrations are: the diffusion of ether molecules from 

 the liquid condition as it evaporates among the molecules of 



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