THE NITROGEN CYCLE 233 



for their production are therefore situated mainly in Scandi- 

 navia, or in mountainous districts where water power can be 

 readily utilised. The amount of nitrogen recovered by these 

 artificial processes is, in the aggregate, of small account, com- 

 pared with the silent but widely active processes of nature. 

 The discovery of the natural process by which the apparent 

 loss of nitrogen is made good is due to the researches of two 

 German investigators, Hellriegel and Wilfarth. It will be of 

 interest at this point to follow their discovery to some 

 extent by making certain actual observations, if the season 

 of the year permits. 



If a fairly well-grown plant belonging to the Leguminacese, 

 e.g., an ordinary garden sweet pea, be carefully pulled up by 

 the roots and the latter examined, if necessary with a pocket 

 lens, a number of little nodules will be observed on the rootlets 

 (see Plate II (i) ), which on pressing will exude a milky juice. 

 If a microscopic preparation is made of this juice and it is 

 examined under a high-power microscope, numerous bacteria 

 will be found to be present. Hellriegel and Wilfarth found 

 that plants, such as the sweet pea, were capable of growing in 

 a sterile soil free from nitrogen, if this soil were treated with 

 an extract of earth in which plants of the same family had been 

 previously grown. The addition of this extract determined 

 the development of the root nodules. They concluded that 

 the nodule bacteria in some way assisted the plant to absorb 

 its nitrogen from the air. Their conclusions were confirmed 

 by Breal, who compared the growth of rootlets of lupin, 

 inoculated directly with the liquid contents of a root- 

 nodule, with the development of similar rootlets which 

 had not been inoculated. The growth, and the percentage 

 of nitrogen in the resulting plant, was much greater 

 in the former than in the latter case. It is common 

 knowledge that peas and beans are the chief sources 

 of nitrogen in a vegetable diet; we thus see how by the 

 action of these organisms, in assisting plants of this character 



