BACTERIA IN THE SOIL 147 



plant will perish if nitrogen does not exist in some combined 

 form in the soil. Nitrates and compounds of ammonia are 

 widely distributed in nature, and it is from these bodies 

 that the plant obtains, by means of its roots, the necessary 

 nitrogen. 



Until comparatively recently it was held that plant life 

 could not be maintained in a soil devoid of nitrogen or com- 

 pounds thereof. But it has been found that certain classes 

 of plants (the Leguminosce, for example), when they are 

 grown in a soil which is practically free from nitrogen at the 

 commencement, do take up this gas into their tissues. One 

 explanation of this fact is that free nitrogen becomes con- 

 verted into nitrogen compounds in the soil through the 

 influence of micro-organisms present there. Another ex- 

 planation attributes this fixation of free nitrogen to micro- 

 organisms existing in the rootlets of the plant. These two 

 classes of organisms, known as the nitrogen-fixing organisms, 

 will require our consideration at a later stage. Here we 

 merely desire to make it clear that the main supply of this 

 gas, absolutely necessary to the existence of vegetable life 

 upon the earth, is drawn not from the nitrogen of the at- 

 mosphere, but from that contained in nitrogen compounds 

 in the soil. The most important of these are the nitrates. 

 Here then we have the necessary food of plants expressed 

 in a sentence : water, gases, salts, the most important and 

 essential gas and some of the salts being combined in nitrates. 



Plant life seizes upon its required constituents, and by 

 means of the energy furnished by the sun's rays builds these 

 materials up into its own complex forms. Its many and 

 varied forms fulfil a place in beautifying the world. But 

 their contribution to the economy of nature is, by means of 

 their products, to supply food for animal life. The products 

 of plant life are chiefly sugar, starch, fat, and proteids. 

 Animal life is not capable of extracting its nutriment from 

 soil, but it must take the more complex foods which have 



