FORMATION AND FUNCTIONS OF THE NODULES 263 



One-and-twenty years later FRANK (III.) showed that the formation of nodules 

 does not occur when the plants are grown in sterilised soil, thus proving that the 

 co-operation of microbes existing in the soil is a necessary factor. This result, 

 conjoined with Woronin's observations, led to the conclusion that the production 

 of the nodules is effected by soil bacteria. Frank's observation, and the con- 

 clusion deduced therefrom, were subsequently confirmed by H. M. WARD (V.), 

 who showed that the nodules are absent in water-cultures of Vicia Faba in 

 sterilised nutrient solutions, but, on the other hand, appear in large numbers if 

 chopped nodules, grown in the ordinary soil, be inserted amongst the root-hairs. 

 This discovery threw a little more light upon the manner in which the nodules 

 are produced, and increased the probability of the assumption that they result 

 from the activity of bacteria which gain access to the root, and there exert a 

 certain stimulance inducing a luxuriant cell-growth. A more intelligent inves- 

 tigation of the importance and mode of action of the nodules thus became possible, 

 and it was then remembered that the Leguminosce are precisely the plants found 

 capable of growing in soil destitute of nitrogen. Hence the obvious idea sprang 

 up that possibly these nodules should be regarded as organs facilitating the 

 absorption of uncombined nitrogen from the air. 



It naturally follows that if this assumed faculty is actually possessed by these 

 growths, a direct connection between the formation of the nodules and the 

 development of the plant as a whole should be traceable, and this was accom- 

 plished by HELLRIEGEL (I.), in conjunction with WILFARTH, in the years 1884 

 to 1886. These workers, as a result of exhaustive investigation of plant-roots, 

 arrived at the conviction that the development of the root-nodules stands in 

 most intimate relation to the growth and assimilation of the whole plant. The 

 number of nodules per plant was found to be the greater in proportion as the 

 development of the latter was more perfect. Papilionaceous seeds (e.g. peas) 

 sown in boxes of sterilised soil devoid of nitrogen, and protected from subsequent 

 infection, perished after nitrogen-hunger set in, but, on the other hand, throve 

 and ripened when the boxes were supplied with a small quantity of an aqueous 

 extract from fertile soil. When the non-nitrogenous soil was watered with 

 a little of the said extract, and then sterilised and covered with a layer of 

 sterilised cotton-wool before planting the seeds, the result was identical with that 

 of the first experiment, i.e. the plants started well, arrived at the stage of 

 nitrogen-hunger after the unfolding of the sixth leaf, and then gradually fell 

 into a consumptive state and perished, barren. By these and many other experi- 

 ments Hellriegel arrived at the incontrovertible conclusion that the absorption 

 of atmospheric nitrogen by the Papilionacece is directly connected with the 

 development (or the presence and activity) of the so-called leguminous nodules, 

 which latter are produced solely by the action of certain bacteria on the roots. 



Hellriegel confined his researches entirely to the Papilionacece, and left out 

 of consideration the other two families, viz., Ccesalpinacece and Himosacece, which 

 form with it the order Leguminosce. The formation of nodules in these two 

 families was subsequently investigated by D. MORCK (I.), with the result that 

 the faculty was discovered in each one of sixty-five species (from thirty-eight 

 genera) examined. These discoveries were supplemented by H. LECOMTE (I.), 

 who, in 1894, proved that nodules are formed in Arachis hypogcea, the earth-nut, 

 a fact already noted by Poiteau in 1852, but afterwards denied by Eriksson. 

 The nodule bacteria of the Soja bean (Sqja hispida) have been described by O. 

 KIRCHNER (II.). On the other hand, neither Morck nor FR. NOBBE (I.) was able 

 to discover the presence of root-nodules on Gleditschia ; and it is found impossible 

 to inoculate this plant by the nodule bacteria of other Leguminosce. Apart from 

 Leguminosce, the organs in question are found (so far as is at present known) 

 only on Alnus, Elceagnus angustifolius, IJippophae, and Podocarpus, all of which 



