The Root Hairs take more than Water out of the Soil.--If a root containing a fringe of root hairs is washed carefully, it will be found to have little particles of soil still clinging to it. Examined under the microscope, these particles of soil seem to be cemented to the sticky surface of the root hair. The soil contains, besides a number of chemical compounds of various mineral substances,--lime, potash, iron, silica, and many others,--a considerable amount of organic material. Acids of various kinds are present in the soil. These acids so act upon certain of the mineral substances that they become dissolved in the water which is absorbed by the root hairs. Root hairs also give off small amounts of acid. An interesting experiment may be shown (see Figure on page 80) to prove this. A solution of _phenolphthalein_ loses its color when an acid is added to it. If a growing pea be placed in a tube containing 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.--Living 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 without certain of these mineral substances can be proved by the growth of seedlings in a so-called nutrient solution.[11] Such a solution contains all the mineral matter that a plant uses for food. If certain ingredients are left out of this solution, the plants placed in it will not live. Footnote 11: See Hunter's _Laboratory Problems in Civic Biology_ for list of ingredients. [Illustration: Effect of root hairs on phenolphthalein solution. The change of color indicates the presence of acid.] Nitrogen in a Usable Form necessary for Growth of Plants.--A chemical element needed by the plant to make protoplasm is nitrogen. 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 obtained 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.[12] Footnote 12: 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 reduced in numbers that they cannot do their work effectively. If, then, the soil is heated artificially or treated with antiseptics so as to kill the protozoa, the bacteria which escape multiply so rapidly as to make the land much richer than before.