262 ELEMENTARY STUDIES IN BOTANY 



bacteria was indicated. Certain very important substances 

 needed by plants, especially substances containing nitrogen, 

 are put into usable form by these bacteria. It was stated 

 that when by removing crops these substances become so di- 

 minished in amount that the soil is said to become " poor," 

 they may accumulate again by letting the poor field " lie 

 fallow " until the bacteria have enriched it, or by a " rotation 

 of crops " by means of which the same result is secured 

 more rapidly. This shows that the soil is the home of a 

 world of bacteria, which by their life processes are putting 

 materials into available form for higher plants. 



In addition to the soil bacteria, there are the thready Fungi 

 called mycorhiza ( 40, p. 61), which become attached to 

 the roots of plants and extend indefinitely through the soil, 

 forming a wide ranging system of tubes through which water 

 may be brought to the roots from regions of the soil far be- 

 yond the reach of the roots themselves. 



The picture of the soil, therefore, is that of a complex 

 physical, chemical, and biological laboratory, full of activi- 

 ties of all kinds. It is this delicate and sensitive laboratory 

 that men undertake to use and know very little about. They 

 seek to " improve " it by putting on all sorts of " fertilizers." 

 Many of the fertilizers do some good ; some of them kill the 

 bacterial life, and then the dead soil must be kept " fertil- 

 ized " ; none of them is used with sufficient knowledge of 

 the needs of the soil. What any given soil needs depends 

 upon so many things that there must be developed soil 

 specialists, who will diagnose soil conditions just as a medical 

 specialist is necessary to diagnose conditions of the human 

 body. Especially is this true for soils that are " sick," for 

 then the soil is just as complex a patient as is a sick person. 



144. Entrance of water. To obtain water from the soil, 

 the root not only often branches profusely, but also develops 

 the root-hairs described above ( 140, p. 256). Only in the 

 younger portions of the root, that is, in the general region of 



