198 PRINCIPLES OF AGRICULTURAL CHEMISTRY 



soluble or insoluble. Thus methyl blue is fixed in an insoluble 

 form in the roots of Azalla, while in the roots of Lemna minor it 

 accumulates in a dissolved form. In both cases the dye is 

 accumulated from very dilute solution, and the cells become dis- 

 tinctly colored. The dissolved substances in the cell sap (sugar, 

 salts of organic acids, potassium nitrate, etc.,) must be present in 

 a non-diffusing form. The presence of nitrates in dead cells 

 does not indicate that they are so present in the living cell, for the 

 non-diffusing substances may decompose immediately on the 

 death of the cell. 



Transpiration Movements. Diffusion alone is a very slow 

 movement. It requires 319 days to transport I mg. the distance 

 of i mm. from a 10 per cent, solution into pure water. Most of 

 the material which enters the roots of a plant, though it must 

 diffuse through the root wall, enters in the current of water after- 

 wards transpired, rather than by diffusion alone. 



Active transpiration must lead to the continuous introduction 

 of new traces of salts by the water current, since backward 

 diffusion is slow. This explains how relatively large amounts of 

 saline materials are sometimes found in plants. Nobbe and 

 Siegert actually found patches of saline incrustation on the leaves 

 of buckwheat and barley when the plants were grown in a one 

 per cent, solution of salts. Soluble incrustations are sometimes 

 found on plants, and calcareous scales are often found on the 

 leaves of many Saxifrage and other plants. Transpiration may 

 also aid in the deposition of silica in the cells of plants. 



Absorption by the Root. Excepting carbon dioxide (and nitro- 

 gen in some cases perhaps) the roots absorb all the compounds 

 used to buld up the plant ; namely, water, hydrogen, and oxygen, 

 and all the other elements in the form of salts. Salts of iron, cal- 

 cium, magnesium, sodium, manganese, potassium, silicon, chlorine, 

 nitrogen, sulphur, and phosphorus are thus taken up. 



Salts in solution are more or less decomposed into ions, which 

 are atoms or groups of atoms charged with electricity. Thus, 

 sodium chloride is broken down into the ions, Na and Cl, sodium 

 sulphate into Na and SO 4 , potassium nitrate into K and NO 3 . 



