164 PLANT Lir-K. 



materials, \\hich are obtained from the medium by which 

 they are Mirioiimled. 'I'lrese sul)stances are a weak watery 

 solution of \arious mineral salts, and a gas, carbon dioxide. 



224. Salts absorbed. — Along with the water which is 

 taken into the plant go various amounts of dissolved material, 

 a considerable portion of which consists of mineral salts. 

 When pjlants grow in humus, or in water or soils containing 

 organic matter, a \ariable amount of ( arbon compounds 

 suited for food may be dissoKed l)y the "\\'ater and be taken 

 up by the plant. To this extent the plant will live as a sapro- 

 phyte, and no doubt many i'leld and garden plants have been 

 bred to require this sort of life. Among the mineral salts 

 the most important are the sails of calcium and magnesium, 

 which are present in all soils, in gieater or less quantity, 

 usually in tlie form of nitrates, phosphates, and sulfates. 

 Compounds of two other indispensalde elements, namely, 

 iron and potassium, are dissolved in soil waters. In the 

 same way at least se\cii additional elements are obtained by 

 plants. Besides these, other compounds to a considerable 

 ntmiber, of no use in forming ixiod, are taken in. Silicon, 

 for example, ^^■hich is found in the ash of almost all plants, is 

 of no value either as a food, or for the manufacture of food, 

 although it jilays an important role in increasing the rigidity 

 of certain jjlants, and in protecting others from injury. 



225. Selective action. — Compounds of these elements 

 exist in the water in various, thougli small, amounts. But 

 they are not taken into the ]ilant in the same proportions as 

 they exist in the water. For each substance presented to the 

 ])lant there is a (crtain degree of concentration at ^\■hich its 

 solutions are absorbed with gieater rapidity than at anv other. 

 Substances wdiich arc utilized by the plant and which, there- 

 fore, disappear as such within it b)' having their chemical com- 

 position alteied or by being stored up in a dilferent form 

 and so removed Irom solution, will enter the ])lant contin- 



