164 PLANT LIFE. 



materials, which arc obtained from the medium by which 

 they are surrounded. These substances are a weak watery 

 solution of various 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 plants grow in humus, or in water or soils containing 

 organic matter, a variable amount of carbon compounds 

 suited for food may be dissolved by the water and be taken 

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

 phyte, and no doubt many field and garden plants have been 

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

 the most important are the salts of calcium and magnesium, 

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

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

 Compounds of two other indispensable elements, namely, 

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

 same way at least seven additional elements are obtained by 

 plants. Besides these, other compounds to a considerable 

 number, of no use in forming food, are taken in. Silicon, 

 for example, which 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 plays an important role in increasing the rigidity 

 of certain plants, and in protecting others from injury. 



225. Selective action. — Compounds of these elements 

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

 they are not taken into the plant in the same proportions as 

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

 plant there is a certain degree of concentration at which its 

 solutions are absorbed with greater rapidity than at any other. 

 Substances which are utilized by the plant and which, there- 

 fore, disappear as such within it by having their chemical com- 

 position altered or by being stored up in a different form 

 and so removed from solution, will enter the plant contin- 



