152 



logically considered, they constitute roots modified by the penetration 

 of an exterior organism. Under no circumstances have we a right to 

 consider them as a natural production of the plant, because, as Praz- 

 moffski has shown, plants that are kept protected from all causes of 

 contamination are always free from them; while, on the contrary, 

 their roots become covered with a multitude of nodosities when 

 plunged into a liquid where a tubercle has been crushed or when they 

 are replanted in any sort of soil that is watered with a similar liquid. 



The artificial infection of the roots of leguminous plants, as enun- 

 ciated a dozen years ago by Prillieux, has been verified by Hellriegel 

 and Wilfarth, Prazmolfski, Laurent, and Break This latter investi- 

 gator has even discovered that we may certainly assure the formation 

 of a tubercle by pricking the root of a leguminous plant with a needle 

 which had been previously inserted into a tubercle growing on 

 another root. 



There remains no doubt of this fact: The nodules of the Legumi- 

 nos£e have a microbian origin. The organism which causes them 

 has received the name Bacillus radicicolu; Laurent places it beside 

 the Pasteuria ramosa, between bacteria proper and the lower fungi. 

 Essentially aerobic in its nature, it resists all freezing and drying; 

 but a temperature of 70° C. is sufficient to destroy it. It has been 

 successfully cultivated in bouillons made of peas, or of beans, sup- 

 plemented with gelatine and asparagine, or even in a solution of 

 phosphate of potash and of sulphate of magnesia, to which is added a 

 little sugar, but without any nitrogenous substance whatever. This 

 organism grows in such liquids, preserving its habitual ramified 

 forms, but without producing any true spores. 



As to the tul^ercles themselves, they have until lately been consid- 

 ered as morbid productions, useless to the plant. Some authors have 

 sought to see in them organs either of reserve or organs for the trans- 

 formation of the albuminous substances necessary for the nutrition 

 of the plant ; others — and this is the general opinion at the present 

 time — look upon them as the result of a symbiosis — that is to say, of 

 an extremely intimate association between the root of the plant and 

 the microbe living with it. entirely difJ'erent, however, from the action 

 of the ordinary parasite. 



Hellriegel and Wilfarth were the first to discover a connection 

 between the development of bacteroidal nodosities and the assimila- 

 tion of gaseous nitrogen by the Leguminosse. After having observed 

 that in a culture of peas the most vigorous plants were alwa3's those 

 that possessed the greatest number of tubercles, these investigators 

 carried out many series of systematic experiments in glass jars con- 

 taining 4 kilograms of quartz sand, to which they added certain of 

 the principal minerals necessary to vegetation, such as phosphoric 

 acid, sulphuric acid, chlorine, potassium, etc., and in certain cases a 

 small quantity of nitrogen in the form of nitrates. 



In these jars, which were exposed to the open air, they sowed bar- 

 ley, oats, and peas. The results were exactly the same as those 

 formerly obtained by Ville and Boussingault. 



In soils destitute of nitrogen the crop of cereals (barley and oats) 

 is nearly nil, but it increases in api)roximate ])ro])ortion to the dose 

 of nitrate added, so that for each added milligram of nitrogen there 

 is an increase of crop equal, on an aver;).ge, to 95 milligrams of vege- 



