474 RELATION TO ENVIRONMENT. 



oxygen come from the CO 2 of the air, while the other substances 

 are in solution in the soil, or in certain compounds, or are brought 

 into solution by the plant, or in the case of nitrogen it is some- 

 times fixed from the air by microscopic plants and made avail- 

 able for plant food. There are other substances also in solu- 

 tion in the soil, and plants often absorb certain substances which 

 are not needed for food, and sometimes substances which are 

 harmful. 



920. Effect on plants. Whenever one or more of these ele- 

 ments are in excess in the soil, or where harmful substances are 

 in solution in perceptible quantities, the vegetation responds by 

 changes, by varying in vigor of growth, or a varying ratio be- 

 tween growth and reproduction, or by a modification of the habit, 

 form, structure, and function. The most marked modifications 

 induced by chemical conditions of the soil are in the case of the 

 alkaline or salt basins, the excess of salt interfering with root- 

 absorption, resulting in a modification of stem and leaf by reduc- 

 tion of transpiration surface, and increase of water-storage 

 tissue. Similar modifications also take place in plants grow- 

 ing in peat moors, where there is an abundance of humus acid in 

 the soil. Soils with an abundance of lime also influence certain 

 vegetation, modifying in some cases certain species so that they 

 take an alpine form. The chemical condition of the soil is 

 directly related to the plant's ability to absorb water. The 

 roots of plants will absorb more water when pure than when it is 

 in solution. For absorbing nutrient salts for every plant there 

 is a most favorable concentration, above which they take up 

 water with difficulty. (The concentration of the solution which 

 plants can take up rarely exceeds 3 per cent, and in most land 

 plants is far below this.) Different kinds of salt solutions affect 

 in a different degree the absorption activity of the plant, for 

 example, sodium chloride (table salt) acts more energetically in 

 retarding absorption than sodium nitrate (saltpeter). Mixing 

 of different salts acts more energetically than a single salt. 



The mineral substances in the soil, as nitrates, phosphates, 

 sulphates, lime, potash, magnesium, and iron oxide, are impor- 



