298 Biology in America 



Tucson, Arizona, whose work we have considered in a previous 

 chapter. Synthetic chemistry may well doff its hat and bow 

 low before the greatest creative chemist in the world the 

 green plant. 



The world today hungers and thirsts after nitrogen. We 

 must have nitrogen to fertilize our fields, in order that we 

 may not starve, and we must have nitrogen to rend asunder 

 the bowels of the earth and lay bare the treasures hidden 

 therein, and we must have nitrogen that we may slaughter 

 our fellow men. So we have cleaned the guano beds of Chile, 

 where the sea fowl have been laying down treasure and stench 

 for years untold. We have dug deep into the nitrate beds 

 of Chile and Peru, and today we are harnessing the water- 

 fall and bidding it harvest for us the nitrogen of the air. 

 Meanwhile the silent plant has been putting man's ingenuity 

 to shame, and in its laboratory working wonders, whereat 

 science well may marvel. Truly should man "consider the 

 lilies of the field." 



But the green plant is not unassisted in the wonders which 

 it works. On the roots of plants of the pea family occur little 

 swellings or "nodules" which are formed by bacteria which 

 have the power of extracting the nitrogen from the air in the 

 soil and using it to build their own bodies. Hence they are 

 known as the "nitrogen-fixing" bacteria. As these bacteria 

 die they give to the soil compounds of the nitrogen which they 

 have taken from the air. Thus it is that peas and their 

 relatives such as beans and alfalfa are such valuable plants 

 for crop rotation, for if a soil from which the nitrogen has 

 been largely exhausted by continuous cropping with grain be 

 planted for a year or two to beans or alfalfa, the nitrogen- 

 fixing bacteria on the roots of the latter will replace the 

 nitrogen and give to the worn-out soil a new lease of life. 

 But what share does the bean or the alfalfa and the bacteria 

 have in this co-operative association ? The latter during their 

 life probably absorb sugars and other substances formed in 

 the leaves of the former and passed down into the roots, while 

 on their death some of the nitrogenous material formed by 

 the bacteria is absorbed by the green plant. The heirs to 

 the riches laid up by these two industrious partners are the 

 plants which follow the peas, beans or alfalfa in rotation. 

 There are other soil bacteria which aid the farmer by changing 

 the ammonia in the soil into nitrites and nitrates, in which 

 form it becomes available as food for the green plants, while 

 vice versa other soil bacteria perform exactly the reverse 

 operation and change nitrites and nitrates into ammonia. 

 All life is a cycle. No sooner does one agency build up than 



