42 Principles of Plant Culture. 
tained by these parts has been absorbed by the develop- 
ing plantlet. The patrimony furnished by the seed is 
quickly exhausted. Whence then comes the food that is to 
complete the development of the plant? Aside from the 
carbonic acid already mentioned (59), several other sub- 
stances are required to build up the plant structure. These 
are almost wholly derived from the soil, through the medium 
of the water absorbed by the root-hairs (101). They must 
all be dissolved by the soil water or they cannot enter the 
plant, for they must pass through the cell-walls, which are 
not permeable to undissolved solid matter. 
61. The Elements regarded as Essential in the Food 
of Plants are carbon, hydrogen, oxygen, nitrogen, potassium, 
calcium, magnesium, phosphorus, iron, chlorin and sulfur. 
Some other elements that do not appear essential are also 
used by plants. All of these elements, except oxygen, are 
absorbed by the plant in the condition of chemical com- 
pounds, as water, carbonic acid and various nitrates, sul- 
fates, carbonates, ete. 
62. The Part Played by the Different Elements. 
Carbon is the chief constituent of vegetable substances and 
forms about half of their total dry weight. Plants obtain 
their carbon almost wholly from the air, in the form of car- 
bonic acid gas, which is a compound of carbon and oxygen. 
The leaves absorb and decompose this gas, retaining the 
carbon and giving off the oxygen. Hydrogen and oxygen 
are obtained by the decomposition of water, which is a com- 
pound of hydrogen and oxygen. These enter into the con- 
struction of nearly all tissues. Nitrogen is one of the con- 
stituents of protoplasm (13). Most plants depend upon 
soluble nitrates in the soil for their nitrogen supply, but 
