Plants in Relation to Man 471 



beans, peas, vetches, alfalfa, and the like, known as legumes, 

 depend upon bacteria for nitrogen which they must have in 

 order to live. It is sometimes stated, but incorrectly, that 

 legumes obtain the necessary nitrogen directly from the air; 

 but the fact is that all such plants are unable to assimilate 

 nitrogen except in certain compounds called nitrates (page 572). 

 The bacteria which live in the nodules of the roots of the legumes 

 take nitrogen from the air and, after utilizing it for their pur- 

 poses, throw it off in waste compounds or nitrates which may 

 be directly absorbed and assimilated by the plants. The roots 

 of such plants, when left in the soil, add to its nitrogen content, 

 insuring thus profitable growth of other plants that are not 

 favored similarly by bacteria. 



Organic matter, such as plant stalks and roots and manure, 

 is converted into plant food by the action of bacteria different 

 from the kind just mentioned. These bacteria, together with 

 molds, yeasts, and other minute organisms (page 552), bring 

 about the decay or putrefaction of such matter. They are 

 dependent, however, upon the proper aeration of the soil ; 

 they also aid in this by causing the decay of roots and other 

 vegetable matter, which leave spaces where the air circulates. 



Of the larger organisms living in the soil and depending upon 

 soil air, we may mention the familiar earthworms, moles, and 

 mice among other underground animals. These, by burrowing 

 through the soil and by other means, contribute to its aeration 

 and fertility. 



If air is expelled and kept out of the soil, as it may be, for 

 instance, by water, its absence is soon shown by plants. It 

 is perhaps most easily observed in the case of the leguminous 

 plants. The bacteria supplying these plants with nitrogen 

 compounds are seriously affected or destroyed ; and the plants 

 become sickly, turning yellowish in color, as every gardener 

 has observed. This unhealthy condition is due directly to 

 the absence of the nitrogen products of the bacteria, and 

 indirectly to the absence of soil air. 



