The Various Substances Made by Plants 123 



into the simple compounds which later are worked up to nitrates 

 by the others. It is not yet known how these Bacteria accomplish 

 this crucial first step of nitrification, but the source of the energy 

 is plain; it is supplied by their intense respiratory power, in which 

 they surpass some hundred-fold the larger plants. This fact of 

 the nitrification of soils through the activity of Bacteria is one 

 of the most important in nature. 



It may here occur to the practically-minded reader to ask 

 whether this power of Bacteria to add nitrogen compounds to 

 soils cannot be utilized artificially for the enrichment of poor soils. 

 It can be, and to some extent, has been; and living Bacteria 

 of the suitable sorts have actually been multiplied and distributed 

 for trial by our own Department of Agriculture, and have been 

 offered for sale to farmers both in Europe and America, though 

 the process is not as yet a commercial success. However, in the 

 utilization of the nitrifying Bacteria man was long anticipated 

 by at least one great group of Plants, the Pea Family, or Legu- 

 minosaB, the members of which have actually colonized the nitrify- 

 ing Bacteria upon their own roots, thus making sure that the en- 

 tire product of the Bacteria shall be available to themselves 

 without any loss through drainage or use by other plants. Most 

 people have seen upon the roots of Peas, Beans, and others of 

 this family, the wart-like or pea-like swellings, whose appearance 

 is well shown in the accompanying photograph, (figure 38). 

 These nodules are residences inside the plants occupied by the 

 Bacteria. The connection is mutually beneficial, for the Bacteria 

 receive carbohydrates from the green plants which receive nitrog- 

 enous compounds from them. It is because of the efficiency of 

 this arrangement that the seeds of plants in the Pea Family are 

 richer in nitrogenous food substances than any others; and this 

 latter fact in its turn explains why Peas and Beans are the best 

 of all plant substitutes for meat, which is mostly protein. This 

 relative richness of Leguminosa? in nitrogenous compounds ex- 

 plains also the reason underlying the ancient farming practice 



