DISCOVERY OF TIIK LEGUMINOUS NOIM'UIS 



261 



192. The Discovery of the Leguminous Nodules. 



The source of the free nitrogen accumulated by this class of plants must be 

 sought in the atmosphere alone. 



Formerly the ammonia compounds of carbonic acid, nitrous acid, and nitric acid, 

 c\l ways present in the air (i.e. the rainfall), were considered as likely sources of 

 nitrogen. The quantities of the last-named acid brought down in the rain, in 

 temperate and tropical climates respectively, are given in the following table, 

 drawn up (partly from personal experience) by A. MtiNTZ and Y. MARCANO (I.) : 



The researches and calculations made by HELLRIEGEL (I.), in particular, 

 showed, however, this addition of nitrogen is much too small to deserve all the 

 credit of the enrichment of the soil. Moreover, since the supply is delivered in 

 approximately regular quantity to all the fields in a given district, it would afford 

 no explanation of the fact that, of all these fields (without nitrogenous manure), 

 it is only just those that have been planted with leguminous crops that yield such 

 a surplus of nitrogenous matter. 



Consequently, only one other feasible explanation remains, viz., that the 

 Leguminosce possess the inherent property of absorbing nitrogen from the air, and 

 elaborating it into nitrogenous compounds (albumen, &c.). 



However compulsory this conclusion may be, its recognition by the majority 

 of vegetable physiologists and agricultural chemists was slow. Both classes 

 were under the influence of Boussingault, whose experiments (1837 to 1858) in 

 the cultivation of leguminous and other plants under bell-glasses in soils destitute 

 of nitrogen gave results confirmed by the check experiments of Lawes and 

 Gilbert which seemed to deny the absorption of free atmospheric nitrogen by 

 plants. 



Shortly afterwards, however, attention was directed to the bodies now termed 

 leguminous nodules. These are lateral appendages or swellings on the roots (as 

 shown in Fig. 59), and occur both on the younger and older portions of same, the 

 former being the most thickly infested. In one and the same plant all the inter- 

 mediate stages of this formation, ranging from such as are just barely perceptible 

 to those as large as a pea or hazel-nut, can be found. Their form (Fig. 60) 

 differs in different species, observations on which point have been published by 

 A. TSCHIRCH (I.) and by LAWES and GILBERT (I.). 



The earliest description of the leguminous nodules was given by MALPIGHI (I.) 

 in his book published as far back as 1687, and this observer referred to them as 

 galls, i.e. diseased excrescences, an opinion also shared by P. DE CANDOLLE (I.) 



l i kilo per hectare = 0.89 Ib. per acre. 



