80 



SOIL AND ITS RELATION TO ROOTS 



ing some of this solution the latter will quickly change from a rose 

 pink to a colorless solution. 



A Plant needs Mineral Matter to Make Living Matter. — Liv- 

 ing matter (protoplasm), besides containing the chemical elements 



carbon, hydrogen, oxygen, and nitrogen, 

 contains a very minute proportion of 

 various elements which make up the 

 basis of certain minerals. These are 

 calcium (lime), sulphur, iron, potassium, 

 magnesium, phosphorus, sodium, and 

 chlorine. 



That plants will not grow well with- 

 out certain of these mineral substances 

 can be proved by the gro\vth of seed- 

 lings in a so-called nutrient solution.^ 

 Such a solution contains all the mineral 

 matter that a plant uses for food. If 

 certain ingredients are left out of this 

 I^M WB solution, the plants placed in it will not 



VT^ ^w live. 



i"-^-"^ — -^ r — -^rrzz a — Nitrogen in a Usable Form necessary 

 Effect of root hairs on phenol- for Growth of Plants. — A chemical 

 phthaiein solution. The element needed by the plant to make 



change of color indicates , . . 



the presence of acid. protoplasm IS mtrogeu. The air can 



be proven by experiment to be made 

 up of about four fifths nitrogen, but this element cannot be taken 

 from either soil water or air in a pure state, but is usually ob- 

 tained from the organic matter in the soil, where it exists with 

 other substances in the form of nitrates. Ammonia and other 

 organic compounds which contain nitrogen are changed by two 

 groups of little plants called bacteria, first into nitrites and then 

 nitrates.2 



^ See Hunter's Laboratory Problems in Civic Biology for list of ingredients. 



2 It has recently been discovered that under some conditions these bacteria are 

 preyed upon by tiny one-celled animals {protozoa) living in the soil and are so re- 

 duced in numbers that they cannot do their work effectively. If, then, the soil 

 i? heated artificially or treated with antiseptics so as to kill the protozoa, the bac- 

 teria which escape mxiltiply so rapidly as to make the land much richer than before- 



