CHAPTER XIV. 

 SOME SPECIAL ASPECTS OF NUTRITION OF PLANTS. 



SOURCES OF PLANT FOOD. 



197. 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 elabora- 

 tion of food materials in the plant, the building up of organic 

 compounds used for food and storage, and the assimilation 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 or- 

 ganic compounds (example, those formed by plants, as carbohy- 

 drates, proteids, etc.) and the inorganic compounds* (mineral 

 substances, etc., and those not containing carbon). The essential 

 food constituents or elements in the organic compounds are carbon, 

 hydrogen, oxygen and nitrogen. In the inorganic compounds the 

 essential constituents or elements are calcium, potassium, magne- 

 sium, phosphorus, sulphur and iron^\ though calcium is not 

 essential to the growth of the fungi. 



* The terms organic compounds and inorganic compounds are em- 

 ployed here in the older sense. 



t 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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