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logically considered, they constitute roots modified by the penetration 
of an exterior orgayism. 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, Prazmoffski, Laurent, and Bréal. 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- 
nose have a microbian origin. The organism which causes them 
has received the name Bacillus radicicola; 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 Saleen 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 tubercles 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 different, 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 Leguminose. After having observed 
that in a culture of peas the most vigorous plants were alwe ays 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, ete., 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 approximate proportion to the dose 
of nitrate added, so that for each added milligram of nitrogen there 
is an increase of crop equal, on an average, to 95 milligrams of vege- 
