40 



O LOSS A It r OF 



Pood of Plants. 



Growth implies assimilation of food. The elements of plant- 

 food are ascertained by making a chemical analysis of 

 the plant itself. AVater forms a very considerable per- 

 centage of the whole weight, but is present to a greater 

 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 may be expelled by careful drjang, and if 

 Avhat is then left is burnt, what is called the oryaiiic part 

 of the plant disajipears, and the inorrjanic part (the ash) 

 remains behind. The organic part consists mainly of the 

 elements carbon, hydrogen, oxj'gen, 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 dr?/ 

 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 

 oxj-gen. This is absorbed by tlie 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 by the plant is derived chiefly from the carbonic 

 acid gas and from water. Hydrogen is obtained chiefly 

 by the decomposition 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 

 cour.se, are all the inorganic elements 



Respiration. 



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

 as with animals, oxygen is essential to their existence. 

 Germinating seeds and growing parts require large 

 quantities of oxj'gen. The gas when inhaled is combined 

 with carbon, giving rise to carbon dioxide. This process 

 of oxidation is alwaj's accompanied by evolution of heat. 

 This is well illustrated in the pi-ocess of malting, where 

 damp bai'ley is heaped together. As soon as the grain j 

 begins to sprout oxygen is rapidly absorbed, and a very 

 decided rise of temperature takes place. 



