CHAPTER XIII. 



NUTRITION. 



191. Repair and growth. — Since the body of every plant 

 is constantly wasting away by reason of its own activity, it is 

 necessary that it should be as constantly repaired. It must 

 also, for a considerable time or throughout its whole life, be 

 furnished with material which can be used in the making of 

 new parts. Without an adequate supply of food, therefore, 

 neither repair nor growth is possible. To understand what 

 materials are necessary for repairing waste and forming new 

 parts of the living plant, the constituents of a plant may be 

 determined by chemical analysis. 



192. Chemical composition. — The greater portion of the 

 weight of every plant is found to be water. Of the firmer 

 parts it forms as much as 50 per cent., while of the softer 

 parts it may form 75 or even 90 per cent. The most watery 

 portions of some plant bodies, such as the juicy portions of 

 fruits and the whole body of the algas, may contain only 2 to 

 5 per cent, of solid matter. The solid material, left after 

 driving off the water at a temperature of no° C, is found to 

 consist chiefly of three elements, carbon, hydrogen, and 

 oxygen. The most abundant element in addition to these is 

 nitrogen. If the dry substance be burned these four elements 

 are driven off in gaseous forms, and there remains a white 

 material which crumbles under pressure, the ash. An analy- 

 sis of the ash reveals the presence of sulfur and phosphorus 

 in considerable amounts, and also smaller quantities of the 

 following elements: calcium, magnesium, potassium, iron, 



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