116 Practical Farming 



pod of the clover to the large pod of the pea, did in 

 some way add to the fertihty of the soil on which they 

 grew. Many supposed that they absorbed ammonia gas 

 from the air through their leaves, for it was found that the 

 content of nitrogen in the crop was greater than could 

 be had from the soil. But in recent years this capacity 

 of the leguminous plants for acquiring nitrogen has been 

 the subject of close and long-continued study by leading 

 biologists of Europe and America, and the process is now 

 better understood. That is, we have found out the or- 

 ganisms which enable the clover and peas to get nitrogen, 

 though the exact way in which they do it is yet a matter 

 for investigation, and there are several theories in regard 

 to it. For the farmer in practice it is enough to know 

 that they do get nitrogen for him. The air is made up of 

 a mixture of oxygen and nitrogen, as we have heretofore 

 seen. The nitrogen is not in chemical combination with 

 the oxygen, but is simply mixed with it to dilute the 

 oxygen so that animals can breathe it. It is therefore 

 termed free nitrogen. It was found that whenever there 

 was an increase of nitrogen in the clover, peas, or other 

 pod plants, there were always a number of Httle lumps or 

 nodules on their roots, and when the plants had none of 

 these nodules they failed to increase in nitrogen. It was, 

 therefore, assumed that these nodules had something to 

 do with the work, and scientists began to pay special 

 attention to the structure of the nodules. Careful micro- 

 scopic investigation revealed the fact that these nodules 

 were inhabited by minute living plants of extremely small 

 size. As they differed in form from any bacteria that 

 had before been studied, they were at first termed Bac- 



