CHAPTER IX. 



HOW PLANTS OBTAIN THEIR FOOD. I. 

 1. Sources of Plant Food. 



170. The necessary constituents of plant food. As indicated in Chap- 

 ter 3, investigation has taught us the principal constituents of plant food. 

 Some suggestion as to the food substances is derived by a chemical analysis 

 of various plants. In Chapter 8 it was noted that there are two principal 

 kinds of compounds in plant substances, the organic compounds and the 

 inorganic compounds or mineral substances. The principal elements in 

 the organic compounds are hydrogen, carbon, oxygen and nitrogen. The 

 elements in the inorganic compounds which have been found indispensable 

 to plant growth are calcium,* potassium, magnesium, phosphorus, sulphur 

 and iron. (See paragraphs 54-58, and complete observations on water 

 cultures.) Other elements are found in the ash of plants; and while they 

 are not absolutely necessary for growth, some f of them are beneficial in 

 one way or another. 



171. The carbohydrates are derived, as we have learned, from the CO 2 

 of the air, and water in the plant tissue drawn from the soil; though in the 

 case of aquatic plants entirely submerged, all the constituents are absorbed 

 from the surrounding water. 



172. Food substances in the soil. Land plants derive their mineral food 

 from the soil, the soil received the mineral substances from dissolving and 

 disintegrating rocks. Nitrogenous food is chiefly derived from the same 

 source, but under a variety of conditions which will be discussed in later 

 paragraphs, but the nitrogen comes primarily from the air. Some of the 

 mineral substances, those which are soluble as well as some of the nitrog- 

 enous substances, are found in solution in the soil. These are absorbed 

 by the plant, as needed, along with water, through the root hairs. 



* Calcium is not essential for the growth of the fungi, 

 f For example, silicon is used by some plants in strengthening supporting 

 tissues. Buckwheat thrives better when supplied with a chloride. 



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