THE FOOD OF PLANTS. 
61 
organising forces of vegetable life, and the structures thus built 
up serve for the sustenance of the animal — this cycle has a 
definite origin. In order that the cycle, or chain, which is 
continually repeating itself, should be able to do so, it is 
essential that the last term' of this cycle should be a prepara- 
tion for the first ; that each substance which served as food for 
the living plant should be left in that particular and appro- 
priate form when the life of the animal ceases, or should have 
been previously restored. So we may commence our inquiry into 
the special forms of material which constitute the food of plants at 
one end of the chain or at the other. We may ascertain what 
mineral forms the plant is feeding on, or we may investigate 
and detect the substances which the animal restores to the 
inorganic world during its life or after its death. By both 
methods we are led to the same conclusions, with some slight 
exceptions here and there — exceptions probably more apparent 
than real. 
Beginning with the volatile elements of a plant, we turn 
naturally at first to the organic element 'par excellence^ carbon. 
Dr. Hofmann, indeed, used to speak of organic chemistry as 
the history of the wanderings of carbon ; ” and this aspect 
i of the subject strikes us with peculiar force in the present 
' inquiry. The oxidation of carbonaceous matter into carbonic 
I acid CO25 by animals, and the separation of the combined oxy- 
j gen of this compound by plants, is one of the most important 
j parts of the balance of animal and vegetable life, even serving 
j to distinguish the higher forms, at all events, of the vegetable 
I from those of the animal kingdom. Then, too, the large pro- 
portion in which carbon exists in plants marks it out as their 
I most characteristic element. In round numbers, half the 
; weight of a dried plant is carbon. Now it is pretty generally 
I allowed that it is only in the form of carbonic acid that carbon 
I is taken up by plants. All other carbon compounds, carbonic 
* oxide and marsh gas, with the more complex substances con- 
i taining this element, seem to be either useless or even actually 
I injurious to plant life. The organic matters, for example, con- 
tained in peat and similar soil, rich in substances derived from 
plants, are generally hurtful to the growth of the higher forms 
of vegetable life. They contain carbonic acid, it is true, and 
they continually evolve it, and so far provide nutriment to the 
plant, but they contain in excessive quantity humus or organic 
I mould. They are often said to be sour, and they do in truth 
contain acids, substances intermediate between the original 
materials of the plant and the final products of decay. These 
transitional compounds are always seizing oxygen, but are not yet 
sufficiently oxygenated to constitute plant-food, and they further 
interfere with vegetable nutrition by reducing to useless and 
