CONSTITUENTS OF PLANTS. 177 



CHAPTER XX, 



POOD 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 12 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, hydrogen, 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 carried down 

 into the soil dissolved in rain-water, and the solution is 

 then absorbed by the roots and transmitted by the stem 

 to the leaves, where, 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 chemical! v combined with the other elements to 



