Part III.] OR POISONED BY VEGETABLE GROWTH ? 
159 
of oxygen—consequently, of atmospheric air— 
is necessary for the generation of carbonic acid 
from the humus in the soil; and Priestley and 
Senebier have shown that from carbonic acid 
plants assimilate their carbon (which, loosely 
speaking, the whole of the tree may be said to 
consist of), by decomposing the carbonic acid, 
and giving off the oxygen. I should consider 
this as the cause why roots keep within the 
reach of atmospheric aeration, since the main 
article of the food of trees' is found in that 
district; though Liebig, and a host of modern 
physiologists, follow Priestley, Senebier, and De 
Saussure in thinking that after the first infancy 
of the plant, that is, after the development of 
leaves, it is indebted to the atmosphere only for 
the supply of carbonic acid. But can we doubt 
that the chief growth of plants is from con¬ 
stituents absorbed from the soil, not from the 
atmosphere, when we see the perpetual difference 
of growth of the same plants in the different 
soils of the same parish; that is, in the same 
atmosphere ? 
Liebig supposes plants to assimilate their 
nitrogen by decomposing ammonia, stored in 
soils from rain water, manure, and humus, and 
giving off the hydrogen ; their hydrogen, by 
decomposing water and giving off the oxygen. 
