June t , 1891.] 
fHIfc TR6P)0AL AaRiOULTURISt. 
527 
theless supposed to be brought under contribution in 
various ways. Coming into combination — by tiie 
agency of electricity, within the soil, or within the 
plant ; or under the influence of microorganisms, or 
of other low forms, either within the soil itself, or 
in symbiotic growth with a higher plant. 
In July last. Professor J. H. Gilbert delivered a 
lecture at the Royal Agricultural College, Cirencester, 
in continuation of a series on the results of experi- 
ments at Rothamsted with different crops, taking, 
as his subject, the “Fixation of Free Nitrogen,” and 
giving a description and some of the numerical results 
of experiments made at Rothamsted in 1888, a des- 
cription, and some illustrations of the growth in 1890, 
and also a brief account of the experiments still in 
progress. This lecture, with additions, has now been 
published, and the following abstract may be taken 
as giving the main conclusions: — 
How IS THE Fixation of Nitrogen to be 
Expi.ained '? 
Revising tlie whole of the Rothamsted results 
there can be no doubt tliey are abundantly con- 
firmatory of those obtained by Hellriegel and Wilgarth, 
which are, that plants of the gramineous, the cheno- 
podiaceous, the xiolygonous, and the cruciferous 
families, depend on combined nitrogen sup)plied 
within the soil ; while leguminous xilants do not 
depend entirely on such supjilies. Also, that the 
fact of the fixation of free nitrogen in the growth of 
Leguminos®, under the influence of suitable microbe 
infection of the soil, and of the resulting nodule 
formation on the roots, may be considered as fully 
established. Although, as Sir J. B. Lawes and Dr. 
Gilbert admit, we must obviously know more of the 
nature and mode of life of the organisms which in 
symbiosis with the leguminous plant bring about the 
fixation of free nitrogen, before the nature of the 
action can be understood. 
It is a point of imxiortance that it should be estab- 
lished, as it apixiears to be, that in the develoinuent 
of the xiarasite, the cortex of the root of the host is 
penetrated, and so an intimate connection between 
the two — indeed, a symbiosis — is set ux>. Then there 
is abundant evidence that the nodules are very rich 
in nitrogen. So far as the facts at command go. it 
would seem that, in certain stages of their develop- 
ment, their dry substance may contain a much higher 
Xiorcentage of nitrogen than that of any x>art of the 
growiiig plant itpelf ; and in some cases, even higher 
than in that of the highly nitrogenous leguminous 
seed. 
But w'hether or not it may eventually be established 
that nitrogen is fixed by microbes within the soil 
indexiendently of leguminous growth, there is evidence 
that in soils and subsoils containing organic nitrogen, 
lower organisms may serve the higher x’l^its, by 
taking up, or attacking and bringing into a more 
readily available condition, combined nitrogen not 
otherwise, or only very slowly, available for the higher 
Xflants. For example, it is probable that fungi 
generally derive nitrogen from organic nitrogen ; 
and in the case of those of fairy rings, there can be 
little doubt that they take uxi from the soil organic 
nitrogen, which is not available to the meadow xflants, 
and that, on their decay, their nitrogen becomes 
available to the associated herbage. Then in the 
case of tho fungjis-niantle observed liy Frank on the 
roots of certain trees, it may be sux^xiosed that the 
fungus takes up organic nitrogen, and so becomes 
the medium of the supxfly of the soil nitrogen to the 
plant. 
More xiertinent still, is the action of the nitrifying 
tu-gamsms in rendering the organic nitrogen of the 
soil iind' subsoil available to the higher pli'-nts- 
ft may well bo supposed, therefore, that there may 
be other cases, in which lower organisms may serve 
the higher, by luinginginto a more available condition 
the combined nitrogen already existing, but in a com- 
puratively inprt state, in soils and subsoils. 
As to tli.o undoubted fixation of free nitrogen in the 
growth of leguniiudus cr 0 ))s under the inlluence of 
suitable microbe infection, and of the dcvelopiuent of 
jiocluics yii UiO ivbts of the x'litnts, the alternate 
Beem to oe: — 
1. That the plant is enalfled, under the conditions 
of the synibiosis, to fix the free nitrogen of the atmos- 
phere by Its leaves. 
2. That the organisms become distributed within 
the soil, and there fix free nitrogen, the resulting 
nitrogenous compounds becoming available as a source 
01 nitrogen to the roots of the liig'her plants. 
'*■ That free nitrogen is fixed in the course of 
the development of the organisms within the nodules 
and that the resulting nitrogenous compounds arc 
absorbed and utilised by the host-plant. 
Tlr. Gilbert seems to think that the balance of tlic 
evidenoe at xiresent- at command is iiiuoh in favour of 
the third mode of explantation. 
Practicae Importance of the Question. 
In reprd to the practical importance of the newly- 
established source of nitrogen to the Leguniiiiosm 
considered in its^ bearing on the known facts of agri- 
oiiltiiral pi-oduction, and especially on the question 
ot the sources of the nitrogen, not only of Legu- 
minous crops themselves, but of croxis generally it 
may be stated that both agricultural investigation 
and direct vegetation experiment, have clearly shown 
tliat Leguminosie do take up much soil-nitrogen and 
at any rate, in gre.at part, as nitrate. But the 
evidence does not justify the conclusion that tlie 
whole of the nitrogen has been so derived, and while 
the blanaoe of experimental evidence is against the 
suxixiosifcion that the higher xflants growing under 
sterilised conditions can assimilate free nitrogen- 
it is established that, at any rate , in the case of 
some leguminous xflants, tliey may acquire nitrogen 
coincidently with the develoximeut on their roots of 
tubei’cnlar bodies with bacteroid contents; and the 
evidence points to the conclusion that it is the lower 
organisms, and not the higher plants, that fix free 
nitrogen. And further, that w’heii such plants are 
grow'ino- in soil and subsoil containing an abundance 
ol combined nitrogen, loiver organisms may serve 
the higher, at any rate, in xiart, by briiigiii" the 
already combined nitrogen of the soil and subsoil 
into a more readily available condition. 
Summary and Conclusion. 
_ There is no evidence that the leguminous iil-int 
Itself assimilates free nitrogen. The evidence ' nt 
Xiresent at command is in favour of the sniiposition 
that the observed gain is due to the fixation of nitrogen 
in the development of the lower organisms in tiio 
root nodules, and that the nitrogenous compounds 
produced are taken up and utilised by the leguminous 
plant. Further, the develoximeut of the supposed 
nitrogen-fixmg bodies is obviously dependent on due 
miection with organisms essential to tlie settiim up 
of the symbiotic life in the particular leguiiiirious 
plant to lie grown. 
It seems pMbable that, in the growth in xiractical 
agriculture of leguminous crops, such as Clover 
Vetches, Peas, Beans, Lucerne, Ac., at any rate some' 
and in certian cases a considerable proportion of the 
large amount of nitrogen which they contain, and of 
the large amount which they frequently leave as 
nitrogenous residue in the soil for future croiis maybe 
due to free nitrogen so brought into combination bv 
the agency ot lower organisms. 
It has yet to be determined under what conditions a 
greater or less proportion of the total nitrogen of the 
crop will be derived— outlie one hand from nitrot^on 
compounds within the soil, and oh the other Rom 
such fixation. 
fncidentally the question suggests itself— liow far 
tlie failure ot red Clover, or of other leguminous crops 
may be due to the exhaustion of the organisms 
necessary for nodule develoximent, and for the cou- 
sequent_ fixation of free nitrogen, and how far to the 
exhaustion of combined nitrogen in an available con- 
dition, or of the necessary mineral constituents within 
the range of the plant-roots, 
Assuniiug itto be estabiii^ied that a considerable 
proportion of the nitrogen of our leguminous crops 
IS due to hxation under tlie conditions sininoscd. it i.s 
obvious that such a fact will not only serve to explain 
the source of tlie hitherto unaccoiintcd-for amount 
the nitrogen of thoyc crops thcjiiscvc^^, hut it 
