265 



as clover, Vicia, peas and Vicia faba, bear also in fertile soil many nodules, and it is not 

 easy to find specimens wholly devoid of them, unless the soil has been previously 

 sterilised. 



On the roots of Genista anglica and Genista pilosa, growing on poor heath fields, 

 I found after long seeking only very few tubercles, although they and in particular 

 the former, bore many pods with good seeds; the tubercles are, however, never quite 

 absent. When sown in my garden at Delft or brought there as plants, they die after 

 some few years. On the other hand, Genista tinctoria thrives as well at Delft as along 

 the highway of Zutphen to Vorden and at both places bears a small number of 

 nodules. 



For Robinia pseudo-acacia the favourable influence of B. radicicola only on the 

 young plant, has been stated by Nobbe 1 ). As to fullgrovvn specimens on poor heath 

 soil at Gorssel I could after long digging find but few tubercles, while at a small 

 distance, but on somewhat better soil more tubercles occurred, but still so little 

 numerous, that nobody would attribute to them any direct significance for such a 

 large tree, had not the fixation of nitrogen in the tubercles become an inveterate belief. 

 Sarothamnus vulgar is and Ulex europaeus behave in the same way as Robinia. On 

 Phaseolus vulgaris on sandy soils I found but few nodules, and then only on thin 

 rootlets and nearly always enclosed by plant remains; in the pure sand the nodules are 

 very rare. In garden soil at Delft Phaseolus produces no nodules, but it does in a there 

 arranged sandbed; Lupinus luteus and Serradella behave likewise. 



When comparing the various mentioned plants, all noted in agriculture for their 

 power of ameliorating the soil, as they contain in their dry substance nearly double 

 the quantity of nitrogen found in other plants, for example the grasses, we come to 

 the conclusion that only for lupine and serradella the number and weight of the tuber- 

 cles is of some significance in regard to the whole weight of the plant. For other 

 species they are of so little volume that even if within them free nitrogen were fixed 

 with great intensity, only an extremely little quantity of fixed nitrogen could be ex- 

 pected, whilst in reality this amount is very considerable. Hence the theory, at present 

 generally accepted, after which the fixation takes place in the nodules only, requires 

 reconsideration. Also other experiences make this reconsideration necessary. But 

 previously a few remarks on the isolation of the bacteria from the nodules and from 

 other materials, and on the question of their specificity . 



An very convenient medium for isolation was already described in 1888 2 ), namely 

 pea leaves- or clover-extract-gelatin with 2% cane sugar. B. radicicola grows thereon 

 in soft, non-liquefying colonies, while B. ornithopodis from Ornithopus perpusillus, 

 O. sativus or Lupinus luteus, when isolated in the autumn or in March, liquefy some- 

 what, as does B. herbicola*}. 



As a solid medium, poor in nitrogen compounds, I recommend a plate of: Tap- 



') Hiltner I.e. Also Biisgen, Bau und Leben unserer Waldbaume, 2. Aufl., 

 pag. 246, 1917. 



2 ) Botan. Zeitung 1888, pag. 764. 



3 ) Occasionally a great number of colonies of B. herbicola are obtained from the 

 tubercles; whoever is unacquainted with this species may make mistakes in the isolation 

 of B. radicicola. But even with this knowledge the isolation of serradella- and lupine- 

 bacteria is difficult. Good descriptions of these forms do not exist. 



