CHAPTER XXIV 

 ABSORPTION OF MINERAL SALTS 



The dry matter residue remaining from any plant tissue after desiccation 

 in an oven can be separated by a simple analytical procedure into a com- 

 bustible fraction, representing organic matter, and an incombustible fraction 

 called the ash (Chap. XX). The latter corresponds roughly to the mineral 

 salts which have been absorbed from the soil, but does not include any nitro- 

 gen since this element passes off in the combustion process along with car- 

 bon, hydrogen and oxygen. The mineral elements do not occur in the ash 

 in the pure state, but mostly as oxides. The actual values obtained for the 

 ash content of a plant tissue depend upon the ignition temperature used. A 

 portion of some of the mineral elements present is often lost by sublimation 

 or vaporization. This is especially likely to happen to chlorine and sulfur, 

 but potassium, calcium, phosphorus, and perhaps other elements are some- 

 times lost in this way. Hence the ash content of a tissue furnishes only a 

 rather crude measure of the mineral salt content of that tissue. 



The role of nitrogen in the metabolism of plants is discussed in Chap. 

 XXVI, but from the standpoint of the mechanism of absorption, nitrogen 

 will be included among the mineral elements. 



The total ash content of plant tissues and organs varies from a fraction 

 of I per cent to 15 per cent or even more of the dry weight of the plant 

 material. Fleshy fruits and woody tissues are usually low in ash content, 

 often yielding less than i per cent, while the ash content of leaves is usually 

 relatively high, often exceeding 10 per cent. Tobacco leaves, for example, 

 contain on the average about 12 per cent of ash on a dry weight basis. The 

 ash content of other plant organs visually lies somewhere between these two 

 extremes. 



Elements Found in Plants. — It is probable that there is not a single 

 one of the chemical elements which is not found at least in traces in some 

 species of plant, under certain conditions. Actually about 40 of the known 

 elements have been identified as occurring in plants by chemical analysis. 

 The list of these elements includes carbon, hydrogen, oxygen, nitrogen, sul- 

 fur, phosphorus, potassium, calcium, magnesium, iron, chlorine, silicon, alu- 



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