brought, into contact with the oxygen of the air--which also passes
into the blood through the cell walls of the lungs--and a form of
combustion takes place, the heat generated serving to warm the body
while the carbon dioxid passes back into the lungs and is exhaled
into the open air.

By these circulation processes the supply of carbon dioxid in the
atmosphere is renewed and maintained without any special effort on
the part of man. Hydrogen is one of the elements of which water is
composed. Water is taken into the plant through the roots, carried
through the stems to the leaves, and there, under the influence of
chlorophyll, sunlight and the life principle, the carbon, oxygen and
hydrogen are made to unite into some of the most important plant
compounds, such as the sugars, which are later transformed into
starch and fiber.

Though these three elements constitute the larger part of the mature
agricultural plant they are no more necessary for plant growth than
the seven which are supplied by the soil. Iron is one of the
essential elements of plant food; but the amount required by plants
is so small and the amount contained in the soil is so large that
soils have never been known to become deficient in iron. Though
sulfur is found in plants in very appreciable amounts and is known
to be essential to plant growth, it is evident that plants do not
need so much sulfur as they often contain, some of it being taken up
and merely tolerated, as is the case with all of the sodium and
silicon found in plants, neither of these being required for normal
growth, although commonly found in plants in very considerable
amounts. The supply of sulfur in normal soils is not large; but,
with the combustion and decay of organic materials--coal, wood,