45 
THE SOILS OF FLORIDA. 
of many other minerals, among which are the hydrated minerals, 
limonite, gypsum, and others. It is also a constituent of organic 
matter. The amount of hydrogen in the earth's crust to a depth of 
ten miles is estimated at .22 per cent. The amount in the ocean is 
10.67 per cent. The amount of hydrogen used by plants is consid¬ 
erable, the kernel of corn being 6.4 per cent, hydrogen. The hydrogen 
used by plants is derived from the water absorbed by the roots. 
From the roots, the water passes through the stem to the leaves. In 
the leaves under the influence of sunlight till? water is broken up 
into its constituent elements, hydrogen and oxygen, and is incor¬ 
porated into the organic structure of the plant. 
Water is thus doubly essential to plant life, since in addition to 
serving as an essential food it also acts as a carrier of food and prob¬ 
ably for other purposes. All those elements already described as 
derived from the soil are carried to the plant in solution in the 
water which enters through the roots and is evaporated from the 
leaves. The evaporation of water from the leaves probably prevents 
an injurious rise of temperature. The amount of water thus passing 
through the plant is considerable. Important experiments in this 
connection were carried on in Wisconsin by King. Several crops 
were used in these experiments of which oats and corn may here be 
mentioned as illustrations. Seven trials on oats indicated an aver¬ 
age of 557.3 pound of water evaporated per pound of dry matter 
formed. The average for eight determinations on corn was 275.6 
pounds of water evaporated, per pound of dry matter formed.* 54 
The water thus passing through the plant and acting mechan¬ 
ically as a carrier of food and for other purposes is not to be con 
fused with the much smaller amount of water that is decomposed in 
the leaves and incorporated into the plant. 
Oxygen :—Oxygen is the most abundant element, forming about 
one-half of all known terrestrial matter. It exists free as a gas in 
the atmosphere of which it makes up about 23 per cent. It is found 
in the atmosphere also in combination with carbon as carbon dioxide 
(C 0 2 ), which makes up .04 per cent, of the atmosphere. In combi¬ 
nation with hydrogen it forms water. Oxygen is a chemically active 
element, combines with about all of the other elements and is a con¬ 
stituent in many minerals, the most common of which are water, 
H 2 0, siliceous sand, Si0 2 and the clays. 
Notwithstanding its abundance as a constituent of minerals in 
the soil and as a free gas in the atmosphere, the oxygen used by 
*Wiscon sin Agricultural Experiment Station, 20th Ann. Rpt. p. 320, 1904. 
