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 is 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 m 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 nitx^ogenous organisms to supply 

 nitric acid by oxidation of their dead bodies, all nitric acid, and therefore all tlie 

 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. 



