250 
GARDENING AS A SCIENCE. 
mean to express the most perfect food of plants ; but its credit, and tliat of tlie theory 
of humic nutrition^ are on the wane. For, during a course of culture, liumus is 
added to the soil, and so far from being dissolved and sucked up into the plant, 
the plant confers it to the soil. But Jmmus has its offices, and these it performs 
during its own decomposition, through the agency of air and moisture. When- 
ever M^ater is decomposed, a stream of electricity passes, hydrogen and oxygen are 
liberated, and these unite with the elements of the decomposing humus, and 
produce carbonic acid, hydrocarbon, and perhaps some ammonia. But no pure 
solution of the manure passes into the plant. Liebig thus introduced his theory, 
which appears to acquire ground every day : — 
Woody fibre in a state of decay is the substance called humus. The 
property of woody fibre to convert surrounding oxygen gas into carbonic acid, 
diminishes in proportion as its decay advances, and at last a certain quantity of 
brown coaly-looking substance remains, in which this property is entirely wanting. 
This substance is called mould ; it is the product of the complete decay of woody 
fibre. Mould constitutes the principal part of all the strata of brown coal and 
peat." 
This passage is cited in evidence of the origin of coals : these combustible sub- 
stances differ much in their components, but all are evidently derived from the 
slow decomposition of wood, in which, by the agency of water, some flinty matter 
is introduced, and more or less of bitumen or asphalt. Coal and vegetable 
mould are therefore not dissimilar : but the humus of the chemist is not that sub- 
stance which cultivators understand by the terra — -by it they infer the black 
decaying matter of an old dunghill, or mass of leaves in a state of decay ; and we 
have now to consult the theory of Liebig on the offices of such humus when incor- 
porated with earth. " Humus acts in the same manner in a soil permeable to air. 
It is a continual source of carbonic acid : by loosening the soil which surrounds 
young plants we favour the access of air, and the formation of carbonic acid ; 
and on the other hand the quantity of their food is diminished by every difficulty 
which opposes the renewal of the air. An atmosphere of carbonic acid is therefore 
contained in every fertile soil, and is the first and most important food for the 
young plants which grow in it. In spring, when those organs of plants are 
absent (the leaves), which nature has appointed for the assumption of nourish- 
ment from the atmosphere, the component substance of the seeds is employed in 
the formation of roots. The roots perform the functions of the leaves from the first 
moment of their formation ; they extract from the soil their proper nutriment ; 
namely, the carbonic acid generated by the humus. A plant at this time receives 
its food, both by the roots and by the organs above ground^ and advances rapidly 
to maturity." 
In these detached portions, which are thus collected together, we find the sum 
and substance of the new theory of vegetable nutrition, so far as the roots are con- 
cerned, and we must abandon, in toto, the notion that plants absorb manure in 
