44 Principles of Plant Culture. 



hausted. Whence then comes the food that is to com- 

 plete 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 in 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 Eood of 

 Plants are carbon, hydrogen, oxygen, nitrogen, potassium, 

 calcium, magnesium, phosphorus, iron, chlorin and sul- 

 fur. 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 con- 

 diti6n'of chemical compounds, as water, carbonic acid 

 and various nitrates, sulfates, etc. 



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 

 carbonic acid gas, which is a compound of carbon and 

 oxygen. The leaves absorb and decompose this gas, re- 

 taining the carbon and giving off the oxygen (59). Hy- 

 drogen and oxygen are obtained by the decomposition of 

 water, which is a compound of hydrogen and oxygen. 

 These enter into the construction of nearly all tissues. 

 Nitrogen is one of the constituents of protoplasm (13). 

 Most plants depend upon soluble nitrates in the soil for 

 their nitrogen supply, but those of the natural order to- 



