398 THE FOOD OF PLANTS 



(KPS), whereas oats in soil containing no combined nitrogen develop 

 but feebly. 



The following are a few of the experimental results obtained by Hellriegel 

 and Wilfarth * with Lupinus luteus. The plants were grown in sand almost entirely 

 free from nitrogenous compounds, and supplied with ash constituents. Certain of 

 the pots were infected with root-tubercle bacteria from ordinary soil ; in others 

 the formation of root-tubercles was prevented ; and in (</) a boiled soil extract 

 was added to show that the trace of nitrogen thus obtained was insufficient to 

 appreciably modify the results obtained. 



Total per pot, containing two plants. 



N. added in seed, Q a f n or / oss 



Harvested dry weight. N. present, soil, and soil- / ** 



extract. 



( (a) 38-010 0-008 0-022 + O-Q7K 



Root-tul)crcles present 



1 (*) 33-755 -98i 0-023 + 0-958 



( (c ) 0-989 0-016 0-020 0.004 



No root-tubercles J ; , x 



( (d) 0-828 o-oii 0-022 0-009 



In the presence of nitrates the amount of free nitrogen assimilated 

 diminishes, and apparently when the former are abundant such assimilation 

 may entirely or almost entirely cease 2 , although further research is necessary 

 to clearly establish this fact, and also to determine whether the growth of 

 Leguminosae and of root-tubercle bacteria is retarded by the cessation 

 of the assimilation of free nitrogen. At present it seems that no such 

 retardation is produced, and if so these plants possess greater accommodatory 

 powers than Clostridium Pastcnrianum, which is unable to develop unless 

 its normal functional activity can be exercised. It is possible that different 

 Leguminosae exhibit various degrees of specialization in this respect, as is 

 indicated by certain researches 3 , although sufficient attention may not have 

 been paid in them to the other co-operating factors which may influence 

 the result produced. Thus a lessened number of root-tubercles, or the 

 presence of a less active variety of bacteroid will cause a plant to exhibit 

 a lessened power of fixing nitrogen, and hence will enable the addition 

 of saltpetre to exercise a greater influence upon the growth of the plant 

 than would otherwise be the case. The facts discovered by Hellriegel will 

 retain their fundamental importance, whatever the nature of the symbiotic 

 relationship may ultimately prove to be. It seems at present most probable 

 that the actual fixation of nitrogen is due to the root-tubercle bacteria, for 

 this power has as yet been discovered only in micro-organisms, and it is 

 not surprising that the root-tubercle bacteria should only be capable of 

 energetically assimilating nitrogen under special conditions, such as are 



I Ber. d. Bot. Ges., 1889, p. 141. 



II Cf. Hellriegel, I.e., 1888, p. 149; Frank, I.e., 1892, p. 41; Hiltner, Versuchsst, 1896, 

 Bd. XLVI, p. 161 (alder). 



3 Cf. Hellriegel and Frank, 1. c. 



