46 OLOSSARY OF 



Food of Plants. 



Growth implies assimilation of food. The elements of plant- 

 food are ascertained by making a chemical analysis of 

 the plant itself. "Water forms a very considerable per- 

 centay:e of the whole weijiht, but is present to a p:reater 

 extent in some portions of the plant body than in others. 

 Fleshy roots, for example, may contain as much as 90 

 per cent., while dry seeds contain only about 12 per cent. 



The water maj- be expelled by careful drying, and if 

 what is then left is burnt, what is called the organic pa,rt 

 of the plant disappears, and the inorganic part (the ash) 

 remains behind. The ortranic part consists mainly of the 

 elements carbon, hydrogen, oxygen, nitrogen, and sul- 

 phur ; while the inorganic part contains very small 

 quantities of phosphorus, iron, calcium, magnesium, 

 and potassium. Of all these constituents of the dry 

 plant carbon is the most abundant, amounting to about 

 half the entire weight. 



Sources of Plant-food. 



All the materials just mentioned are obtained from the air, 

 the water, and the soil. There is constantly' present in 

 the air carbonic acid gas — a compound of carbon and 

 oxygen. This is ab.sorlx'd by the leaves of land-plants, 

 and (being soluble) from the water in which they live, 

 by immersed plants. After absorption the gas is decom- 

 posed and the carbon appropriated. The oxygen re- 

 quired bj- the plant is derived chiefly from the carbonic 

 acid gas and from water. Hydrogen is obtained chiell}' 

 by the decomjKJsition of water, and nitrogen from the 

 nitrates and ammonia salts in the .soil. Sulphur, also, is 

 obtained from .salts occurring in the soil, and so too, of 

 course, are all the inorganic elements 



Respiration. 



Plants, like animals, are continually inhaling oxygen ; indeed, 

 as with animals, oxygen i.s essential to their existence. 

 Germinating seetls and growing parts require large 

 quantiti?s of oxj-ien. The gas when inhaled is combined 

 with carbon, giving rise to carbon dioxide. This process 

 of oxidation is always accompanied by evolution of heat. 

 This is well illustrated in the process of malting, where 

 damp barley is heaped together. As soon as the grain 

 begins to sprout oxygen is rapidly ab90rl)ed, and a very 

 decide<l rise of temperature takes place. 



