ABSORPTION OF WATER AND MINERAL SALTS 43 



thin films of water. It is important to note that these films 

 of water connect the water of the soil to the water of the 

 protoplasm and cell sap of the cell. The root hairs are be- 

 tween and around the soil particles and make the water con- 

 tact between the plant and the soil. If the connection of the 

 root hairs and the soil is broken, as happens in transplanting, 

 the plant cannot get water by diffusion until new contacts 

 are made. It is for this reason that the plant must be 

 guarded against excessive transpiration after transplanting. 



Mineral salts must be in a water solution in order to be 

 diffusive. They follow the same law of diffusion as explained 

 for water molecules, that is, they tend to go from where there 

 are more of a certain kind of molecules or ions to where there 

 are fewer of them. Calcium ions may serve to illustrate in 

 general the absorption of mineral ions. The calcium must 

 first be in solution in the soil water. Oyster shells or lime 

 applied to the soil must form a solution to be valuable. As 

 the plant's content of calcium is used, perhaps to form a part 

 of the cell wall or insoluble crystals of calcium oxalate, the 

 concentration of calcium in the water in the cells becomes 

 lower than the concentration in the soil solution around the 

 root hairs. When calcium is more concentrated in the water 

 outside the cell than in the cell it will diffuse into the cell. 

 Since there is a constant use of calcium by a growing cell, 

 it follows that it will be absorbed, that is, it will constantly 

 diffuse into the cell. 



Poisonous ions, if they are present in the soil in solution, 

 may diffuse into the plant in the same way that useful ions 

 enter. It has been suggested that the residue of continuously 

 spraying with copper and arsenic sprays might become 

 dangerous to the plants and the plants might become poison- 

 ous as food. Since these ions would be continuously removed 

 by plant growth and by leaching from the soil, it is hardly 

 possible that such a danger exists. Selenium is an element 

 found in certain soils and is therefore absorbed by plants. 

 It appears to be more toxic to animals than to plants, and 



