2 8 TEXTBOOK OF BOTANY 



tissues and contain immense numbers of a particular sort 

 of bacillus. These bacilli can use nitrogen, which makes up 

 about four-fifths of the air, in manufacturing complex sub- 

 stances and ultimately in building up the living matter of 

 their own cells. The power of using the nitrogen of the air 

 in this way is possessed by 'some other bacteria and possibly 

 by some fungi, but not by green plants. 



Now, nitrogen is necessary to the building up of all living 

 matter, and green plants as a rule obtain it from the soil in 

 the form of the compounds called nitrates. A soil that 

 contains too small an amount of nitrates will raise only 

 poor crops of grain or potatoes or of almost any useful plant. 

 But in the case of clover and related plants,' when the bac- 

 teria in their root swellings die, as they do in large numbers, 

 the nitrogen-containing compounds in the bodies of the 

 bacteria are taken up by the host plant and are used by it as 

 food. Thus clover and alfalfa are able with the help of the 

 bacteria in their roots to use the nitrogen of the air, and this 

 is the reason why crops of these plants can be raised in soil 

 that is poor in nitrates. It also explains why soil is enriched 

 by growing clover or alfalfa upon it, especially if the crop is 

 plowed under, because then the nitrogen-containing sub- 

 stances of the plant are broken down by the soil bacteria 

 into nitrates which can be used by other plants. The root 

 bacteria of clover and alfalfa are not parasites, but are in a 

 sort of partnership with the host plant ; the host supplies 

 a moist place for development and also probably some forms 

 of food for the bacteria, and the bacteria supply nitrogen- 

 containing compounds for the host plant. 



42. Historical Note. — Perhaps the first observation of bacteria 

 was in 1659 by Kirchner, who saw " minute living worms " in putrefy- 

 ing meat and other foods. Leeuwenhoek first described and figured 

 bacteria in a letter to the Royal Society of London in 1683. The 

 modern study of bacteria began with the work of Cohn, published 

 chiefly between Lhe years 1853 and 1872. Pasteur in 1857 found 

 that the souring of milk is due to bacterial action, and he showed the 



