QUESTION OF THE SOURCES OF THE NITROGEN OF VEGETATION. 
67 
a healthy aspect, and grew well. The plants of botlj the second and third row's 
maintained a sickly, brown-red colour. 
The roots of the plants of the first row, which received the sandy-soil extract, were 
thick with large root-nodules, such as are found in the field with normal vegeta¬ 
tion. On the roots of the second series, without any soil extract, no trace of the 
nodules could be found ; and on the roots of the third series, which received the 
extract from the ricli soil, on only one plant wms a single weak nodule observed. 
Other sandy-soil Papilionacese, such as serradella [Ornithopus saliva), behaved as 
the lupins; whilst peas, vetches, and beans, grew^ best in the third row, and red clover 
gave no special result. 
Hellriegel observes that, although the Papilionaceie have the property of turning 
to account atmospheric nitrogen, they nevertheless do take up nitrogen from the soil, 
especially as nitrates ; but he considers it doubtful whetlier such plants can attain to a 
normal development with nitrates alone, and with the exclusion of micro-organisms. 
Finally, he admits that his observations require control and perfecting in various 
directions; and he confines himself for the present, to the simple statement of his 
experimental results. 
In reference to the foregoing results of Hellriegel, we have already said that we 
have not been able to find any record of the experimental details ; and indeed it seems 
doubtful whether determinations of nitrogen were made, either in soils, seed, or 
products of growth.* Nevertheless, such particulars as are given, can leave no doubt 
w'hatever that the products of growth, both of the peas and of the lupins, where 
favourable conditions were provided, wnuld contain very much more nitrogen than the 
seed sown. If, therefore, the washed sand, and the nutritive solutions, were free from 
combined nitrogen, and the conditions of exposure of the experimental pots in free air 
were such as to exclude the possibility of the access of accidental sources of combined 
nitrogen, the obvious explanation is that the nitrogen gained had its source in the free 
nitrogen of the air. 
Then, as to the conditions under which the free nitrogen has been brought into 
combination ? The negative result with the Graminese, the negative result with the 
peas when everything was sterilised, or when the sand was not seeded by the soil- 
extract, the positive result with the peas when the sand was seeded by the humus soil 
extract, the negative result with the lupins when their soils were not seeded, or when 
they were seeded with the same extract as the peas, and the positive result when 
seeded with the extract from the sandy soil where lupins were growing, seem to 
exclude any other conclusion than that the micro-organisms supplied by the soil 
extracts were essential agents in the process of fixation. Further, the development 
of nodules on the roots was, to say the least, a coincident of the fixation. To 
Hellriegel’s conclusions on this point, the objections have been raised,—first that 
the nodules are a result and not a cause of active growth, and that in fact they con- 
* See foot-note, p. 64. 
K 2 
