46 FLORIDA GEOLOGICAL SURVEY—FOURTH ANNUAL REPORT. 
plants is taken from neither of these sources. The oxygen used by 
plants is derived from reactions which occur in the leaves of plants 
when the carbon dioxide of the atmosphere acts upon the water from 
the soil. 
The amount of oxygen consumed in building up the plant struc¬ 
ture is relatively large, for instance, 46 per cent of the kernel of 
corn is estimated by Hopkins to be oxygen. 
Carbon :—Carbon, although of great importance to plant life, 
occurs in relatively small amounts in the earth’s crust. In the 
atmosphere carbon is found, as stated above, in combination with 
oxygen forming carbon dioxide gas. This gas occurs in small 
amounts, making up only .04 per cent, of the atmosphere, and the 
carbon itself constitutes only about .01 per cent, of the atmosphere. 
In the earth carbon occurs pure as graphite and diamond, as car¬ 
bonate in limestones, and marbles, and as carbohydrate in organic 
compounds, coal, oil and gas. Carbon is estimated to make up .2 per 
cent, of the crust of the earth. Carbon is, like oxygen, one of the 
important elements in plant structure, the corn kernel containing, 
according to Professor Hopkins, 45 per cent, of carbon, or nearly as 
much as of oxygen. 
All of the carbon used by plants is obtained from the carbon 
dioxide of the atmosphere. The carbon dioxide (carbon and oxy¬ 
gen) enters through the breathing pores of the leaf. Water (hydro¬ 
gen and oxygen) also enters the leaf, coming from the roots through 
the stem. A chemical reaction occurs within the leaf by which is 
formed an organic compound, H 2 CO. The excess of oxygen in this 
reaction passes off as free gas. This chemical reaction occurs only 
in the light and in the presence of chlorophyl, the green coloring 
matter of plants. The resulting compound is organic, not mineral, 
and represents that most important process, nature’s laboratory for 
the manufacture of organic compounds, by which process, directly 
or indirectly, all life upon the earth is sustained. 
It is worthy of note that all of the carbon used in plant growth 
is derived from the .01 per cent, of carbon in the atmosphere. In 
fact, the carbon of the atmosphere would be speedily exhausted were 
it not for a cycle by which carbon taken from the atmosphere is 
being restored again to the atmosphere. The decay of plants is a 
process of oxidation by which carbon dioxide is formed. In the case 
of herbs and annual plants the cycle is passed through ordinarily 
within one year. At the close of each growing season, much of the 
plant growth of the summer is subject to decay, and in the process 
of decay the carbon of the plant is oxidized and is returned to the 
