CHAPTER XV. 

 SOME SPECIAL ASPECTS OF NUTRITION OF PLANTS. 



SOURCES OF PLANT FOOD. 



199. The nutrition of plants includes a study of the 

 sources of plant food, the methods of absorption and transport 

 of the food material, the chemical processes (metabolism) in the 

 elaboration of food materials in the plant, the building up of 

 organic compounds used for food and storage, and the assimila- 

 tion of material into new plant substance and structures which 

 enable the plant to grow and reproduce itself. While certain 

 plant foods may be derived under a great variety of forms, there 

 are certain essential constituents of plant food occurring in the 

 various compounds. There are two general classes of plant food, 

 the organic compounds (example, those formed by plants, as 

 carbohydrates, proteins, fats, etc.) and the inorganic compounds 

 (substances not containing carbon). The essential food con- 

 stituents or elements in the organic compounds are carbon, hydro- 

 gen, oxygen, nitrogen, phosphorus, sulphur and iron. In the inor- 

 ganic compounds the essential constituents or elements are 

 calcium, potassium, magnesium, phosphorus, sulphur, hydrogen, 

 oxygen, nitrogen and iron* though it is said calcium is not essen- 

 tial to the growth of the fungi. 



* There are a number of other elements which are not essential constitu- 

 ents of food, but are of use to the plant. For example, silicon (flint, most 

 kinds of sand, sandstone, etc., are oxides of silicon) strengthens the stems 

 of the grains and grasses, and is found in great abundance in the stems of 

 the scouring rushes (Equisetum) which often grow where there is an abun- 

 dance of sand in the soil. Many substances are found in plants which are 

 useful perhaps in protecting the plants from certain of their enemies by 

 rendering them distasteful or poisonous, and many other substances are 

 found which do not appear to be of any use. 



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