44 Principles of Plant Culture. 



at first so plump, have shriveled to a mere fraction of 

 their former size. They have shriveled because the 

 food contained by these parts has been absorbed by the 

 developing 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 (58), 

 several other substances 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 (100). They must all be dissolved in 

 the soil water or they cannot enter the plant, for they 

 must pass through the cell-walls, which are not perme- 

 able to undissolved solid matter. 



60. The Elements regarded as Essential in the Food 

 of Plants are carbon, hydrogen, oxygen, nitrogen, po- 

 tassium, calcium, magnesium, phosphorus, iron, chlorin 

 and sulfur. Some other elements that do not appear 

 essential are also used by plants. All of these elements, 

 so far as they serve as food, are absorbed by the plant 

 in the condition of chemical compounds, as water, car- 

 bonic acid and various nitrates, sulfates, etc. 



61. The Part Played by the Different Elements. Car- 

 bon 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 carbonic acid gas, which is a compound of car- 

 bon and oxygen. The leaves absorb and decompose this 

 gas, retaining the carbon and giving off the oxygen 

 (58). Hydrogen and oxygen are obtained by the de- 

 composition of water, which is a compound of hydro- 



