64 NUTRIENT GASES. 
not absorbed from the surrounding air, although, as is well known, the atmosphere 
contains nitrogen to the amount of 79 per cent of its volume. There can be no 
doubt that though nitrogen permeates the cell-walls of an air-encompassed plant 
much less readily and quickly than carbon-dioxide, yet it is carried from the atmos- 
phere into the ventilation-spaces of green foliage-leaves, and further through the thin 
cell-walls into the laboratories of the protoplasts, where one would expect it to be 
worked up in the same way as carbonic acid. The most careful experiments have 
determined, however, that it is not turned to account in this form by the proto- 
plasts, but that on the contrary it 1s given back unused to the air, and only such 
nitrogen as reaches the interior of plants in combination with other substances is of 
any service there. 
The principal sources of the nitrogen required by plants are nitrates and 
ammoniacal compounds absorbed from the ground; but nitric acid and ammonia 
themselves, of which there are traces in the atmosphere and in water, must not be 
overlooked. The quantity of nitric acid im air is, it is true, even less than that 
of carbon-dioxide; but just as the small amount of carbon-dioxide can be absorbed 
from the air with highly productive results, so may also the still smaller proportion 
of nitric acid be turned to account. The sources of nitric acid are dead organic 
bodies as they decompose and become oxidized. In many ways the process of 
formation of nitric acid from decaying bodies may take place so as to produce 
ammonia in the first place and from it nitric acid. It would seem possible, though 
it is an unproved assumption, that in places where dead bodies of plants and animals, 
vegetable mould, manure, and such things are undergoing oxidation, that is to say, 
in woods and fields, the small quantities of nitric acid that are given off are imme- 
diately taken up by the plants growing there. It must be borne in mind that plants 
behave with reference to what is necessary or useful to them like a chancellor of 
the exchequer preparing his budget; they take these things where they find them. 
The question has been raised, too, as to the source from which the first plants 
that appeared on the earth were able to obtain nitric acid. We are obliged to 
assume that, at that time before the existence of nitrogenous organisms to supply 
nitric acid by oxidation of their dead bodies, all nitric acid, and therefore all the 
nitrogen used in the nourishment of plants, was generated by thunder-storms. We 
know that nitric acid is formed in the air on occasion of electric discharges and is 
deposited on the earth together with rain and dew. This source of nitric acid is 
not yet exhausted, and even at the present day it no doubt plays the same part as 
in the ages long past at the commencement of all vegetable life. 
If nitric acid is used by protoplasts, in the building up of the highly important 
albuminous compounds, it is broken up in a manner similar to the decomposition 
of carbonic acid to form carbohydrates, that is to say, oxygen is separated out. 
In this case, however, sunlight and, therefore, chlorophyll are not immediately con- 
cerned. Moreover, the oxygen that is set free is not eliminated, but is used in the 
manufacture of other compounds in process of formation in the plant, probably in 
that of vegetable acids. 
