22 FEEDING 



the ground. Plants do not have to go about seeking what 

 they may devour. The simple elements which form their 

 food flow around them. Therefore plants are immobile, 

 sessile, as it is sometimes termed. The food comes to them just 

 as the roast geese in Heine's description of heaven perpetually 

 flew up to the angels offering them tureens of delicious soup. 

 That plants are, so to speak, bathed in their food explains 

 the fact that they exhibit the greatest possible surface to the 

 air above and to the soil below. 



The food that is taken in by plants and animals is often 

 compared to the coal which is the source of the energy of the 

 steam engine. But we shall see that there is more in it than 

 that. Like the coal in the engine, the food is burned up inside 

 the body, and so produces the energy which is turned into 

 work and heat, and thus enables us to walk and think and 

 labour and breathe — in fact, it is the source of the power that 

 keeps the plant or animal, as a machine, going. But food in 

 a jDlant or animal does more than provide energy. It is used 

 in rej^airing the wear and tear of the machine, and if the 

 resemblance were complete the coal in a steam engine would 

 be able to replace worn-out parts. This, of course, it cannot do. 



For an animal to be in health its food must contain proteins, 

 hydro-carbons, carbo-hydrates, salts, and water. Proteins are 

 in the main supplied by flesh foods, the hydro-carbons by 

 fats, and the carbo-hydrates by sugar or starch. The necessary 

 salts of various kinds are contained in the above-mentioned 

 foods. 



We have said that plants can live on simple binary com- 

 pounds. They can build up their protoplasm out of inorganic 

 material only. But there is always an exception, and Fungi 

 (mushrooms, mildew^s, moulds), which are devoid of chloro- 

 phyll, have, like the animals, to obtain their energy and food 

 by absorbing organic compounds. This is true, at any rate, as 

 regards their carbon. Carbon that fungi take in must be taken 

 up from organic matter although they can, like green plants, 

 obtain their nitrogen from inorganic salts. They are thus in 

 their feeding habits half-way between plants and animals, 

 and this method of feeding is called saprophytic. They have 

 to live on or near to organic food material. Thus you find 



