348 
the substances given it as food, he says, “a superabundance of carbon 
in the state of carbonic acid conveyed through the roots of plants, with¬ 
out being accompanied by nitrogen, cannot be converted into gluten, 
albumen, wood, or any other component part of an organ ; but either it 
will be separated in the form of excrements, such as sugar, starch, oil, 
wax, resin or gum, or these substances will be deposited in greater or 
less quantity in cells. We know that the starch or potatoes increases 
when the soil contains much liamus, but decreases when the soil is ma¬ 
nured with strong animal manure, although then the number of cells 
increases, the potatoes acquiring in the first place a mealy, in the second 
a soapy consistence.” Late varieties of potatoes do not usually com¬ 
mence the formation of tubers till the growth of their stems is nearly 
complete and their leaves matured. The tubers are formed during the 
heat of summer chiefly ; it is therefore clear, if the carbonic acid gene¬ 
rated by the decaying humus or vegetable matter in the soil, is absorbed 
by the roots and the carbon deposited in the tubers as starch, that plants 
do not obtain their carbon during summer exclusively from the atmos¬ 
phere ; and we may also infer, that providing a soil was rich in carbonic 
acid, and that substances were also present affording nitrogen to the 
growing plants, then the carbonic acid of the soil, instead of being stored 
up as starch, &c., might be employed in the formation of wood, or glu¬ 
ten, or other substances in which nitrogen is an essential constituent. 
Oxygen exists in the air as a gas; it forms about 21 per cent, of the at¬ 
mosphere—eight out of nine lbs. of water consist of oxygen. It enters 
largely into the composition of rocks, and forms numerous combinations 
with many alimentary substances. It is the gas which supports animal 
life, combustion, &c. It is said to be appropriated by plants chiefly in 
combination with hydrogen as in water. Hydrogen forms one-ninth of 
the weight of water; it exists in the atmosphere, in small quantities, 
combined with nitrogen. Sulphuretted hydrogen and hydro-carbonates 
may afford hydrogen to plants. Nitrogen forms seventy-nine per cent, 
of the atmosphere ; it is doubted whether plants can obtain nitrogen di¬ 
rectly from the air, it is found in the soil and the air in combination with 
hydrogen forming ammonia. Ammonia is produced by the decomposi¬ 
tion of animal and vegetable bodies, especially the former ; it is consid¬ 
ered to be the chief source whence plants derive their nitrogen, a most 
