FORMATION OF THE NODULES. 343 



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 conclusion deduced therefrom, were subse- 

 quently 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 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 pro- 

 bability 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 

 investigation of the importance and mode of action of the nodules 

 thus became possible, and it was then remembered that the Legu- 

 minosce 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 for- 

 mation of the nodules and the development of the plant as a whole 

 should be traceable, and this was accomplished 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 experiments Hellriegel arrived at the 

 incontrovertible conclusion that the absorption of atmospheric 

 nitrogen by the Papilionacece is directly connected with the de- 

 velopment (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., Gcesal- 

 pinacece and Mimosacece, which form with it the order Leguminosce. 



