XVli 
been exposed through the medium of the leaves to the action of 
the atmosphere. 
These elaborated matters and the cellular substance itself are 
all formed of oxygen, hydrogen, carbon, and nitrogen, combined 
in various proportions, Carbon, the most abundant element in 
the solid parts of all vegetables, is derived chiefly, if not entirely, 
from the small quantity of carbonic acid existing in the air; ni- 
trogen, which only enters into the composition of a few vegetable 
products, is obtained from the nitrates and ammoniacal salts pre- 
sent in the soil; while the two other elements are probably due to 
the decomposition of a portion of the water taken up by the root, 
the superfluous oxygen being exhaled by the leaves during the 
day. Thus plants form a necessary part of the great cycle of 
Nature, absorbing those gaseous particles produced by the respi- 
ration of animals and the decay of organic matter, and restoring 
to the air, in a purified state, that principle which is necessary to 
the support of all animate being. 
Water forms a considerable portion of growing vegetables, and 
is necessary to their existence. It is chiefly drawn up by the 
fibres of the root, though some plants doubtless absorb much 
from the atmosphere. Being required in very varying proportion 
by different species, the amount present in the soil has a great 
influence upon its vegetation. Some plants will only grow entirely 
submersed, others flourish im the stagnant bog or peat-moss, 
some require the moist but well-drained hill-side, and a few are 
found in the most arid localities, where occasional dews form the 
only source of moisture. 
In addition to these elementary components, vegetables always 
contain a considerable quantity of various other substances of in- 
organic origin. Potash and soda are the most prevalent of these ; 
they are present in all plants, the former alkali existing in by far 
the greater proportion in most inland species, while an abundance 
of the latter is characteristic of those growing near the sea, bemg 
d 
