eONSTITUENTS OF PLANTS. 177 



CHAPTEE XX 



FOOD OF PLANTS — CHEMICAL PROCESSES MOVEMENTS OF 



WATER PHENOMENA OF GROWTH. 



306. The materials of which the substance of a plant 

 is made up are various, and some of them occur in far 

 larger quantities than others. Water forms a very 

 considerable percentage of the whole weight, but is 

 present to a greater extent in some portions of a plant 

 than in others. Fleshy roots, for example, may contain 

 as much as 90 per cent, of water, while dry seeds contain 

 only about 1 2 per- cent. 



307. The water may be expelled by careful drying, 

 and if what is left is. then burnt, what is called the 

 organic part of the plant disappears, and a small quantity 

 of ash remains behind. The organic part is found to 

 consist mainly of carbon, hydrogeii, oxygen, nitrogen, 

 and sulphur ; while the inorganic part (or ash) contains 

 very small quantities of phosphorus, iron, calcium, mag- 

 nesium, and potassium. All these materials are obtained 

 from the air or the soil. There is constantly present in 

 the air carbonic acid gas, a compound of carbon and 

 oxygen, which is exhaled from the lungs of animals, and 

 which is always found wherever wood or coal, or carbon 

 in any form, is being burned. This gas is absorbed 

 directly from the air by the leaves of land-plants, and 

 (being soluble) from the water in which they live by 

 immersed plants. In the presence of chlorophyll and in 

 sunlight the gas is decomposed into its carbon and 

 oxygen. The excess of oxygen is then exhaled and the 

 carbon chemically combined with the other elements to 



