QUESTION OF THE SOURCES OF THE NITROGEN OF VEGETATION. 
G5 
Total above-ground 
dry substance. 
Seed 
produced. 
grams. 
grams. 
1884 .... 
28-483 
13-947 
1885 .... 
27-816 
11-710 
1885 .... 
33-147 
12-426 
1886 .... 
20-372 
8-956 
To convey an idea of what 33 grams of dry produce means, he states that with 
barley that amount was never obtained even witli the addition of nitrates. So far as 
we know, he has not estimated the amount of nitrogen in the produce,which, however, 
would be large ; and as to the source of it, he says it is obviously from the air. 
The quartz sand was washed many times, the nutritive mixture given contained no 
compound of nitrogen, and the plants were watered with distilled water (the first 
third of the distillate not being used). Hence he considered the supposition that acci¬ 
dental impurity was the source of the nitrogen to be out of the question, especially 
when the amount of the produce is considered. Further, the constant failure of the 
Graminese under exactly the same conditions, afforded direct proof that the soil was 
not the source of the nitrogen. He concludes :—• 
The Papilionaceae are distinguished from the Graminese in not being dependent on 
the soil for their nitrogenous food. The sources of nitrogen which the atmosphere 
affords have, for these plants, the highest importance. They alone can suffice to bring 
them to a normal or full development. 
To determine how far the combined nitrogen in the atmosphere was the source, he 
made a series of 4 experiments, in each of which a pot of peas was enclosed under a 
glass shade, and a constant stream of air was passed through ; in No. 1, without first 
washing the air, but in Nos. 2, 3, and 4, the air was washed to remove all nitric acid 
and ammonia. The result was, that the growth was as good with the washed as 
with the unwashed air. There only remained, therefore, the supposition that the 
Papilionacem have the power of utilising free nitrogen. He accepts the conclusion of 
Boussingault that the Papilionacese cannot directly assimilate free nitrogen ; but the 
possibility of an indirect action is not thus excluded. In the first place BEUTHELOxhad 
shown that bacteria abound in the soil, and possess the power of bringing free nitrogen 
into organic combination; and secondly the nodules found on the roots of normally 
growing Papilionaceae are full of bacteria. 
The author has often observed that when peas are grown in a nitrogen-free soil, the 
growth is quite normal, and the colour of the leaves quite healthy, until the reserve 
material of the seed is used up. Growth is then arrested, and the leaves become pale 
or yellow; but after a shorter or longer time, they regain their green colour, a second 
period of growth begins, and it continues to the end. In a series of parallel experi¬ 
ments, however, some plants will develop full, and others only a very limited growth. 
* See foot-note, p. 64. 
MDCCCLXXXIX. —B. 
K 
