FEEDING 77 



Us surroundings. This we can illustrate in the case of 

 each of the " vital functions." 



(i) Feeding. — In the case of amoeba, what we call 

 the. food of the animal is the living prey which it engulfs. 

 The food of the protoplasm, however, consists of the 

 products of digestion of the prey which can be directly 

 incorporated in the protoplasm, or can be broken down 

 to set free energy without such incorporation. Similarly, 

 in the case of a higher animal, the food of the animal 

 is the animal or plant, or the part or product of the 

 animal or plant, which the animal eats ; the food of 

 the protoplasm of its living cells consists of tljie digested 

 products of the original food which have passed into 

 the blood and can be absorbed and incorporated into 

 their structure by the living cells, or broken down 

 into simpler substances so as to liberate energy. 



In a green plant the food of the plant as a whole 

 consists of the water and inorganic salts absorbed by 

 the roots from the soil, and of the carbon dioxide 

 absorbed by the leaves from the air. The food of the 

 living protoplasm of the plant cells, on the other hand, 

 consists of the sugars and proteins built up from these 

 simple substances taken in by the plant. In the case 

 of a germinating seed the food is obtained directly from 

 reserve stores — starch or fats, which are converted into 

 sugar, and proteins which are converted into a soluble 

 form — that are packed away in the seed. Here, it will 

 be noted, the foods of the young plant are of exactly 

 the same chemical nature(^ as the foods of an animal, 

 and they can be used, and are in fact largely used, as 

 food by animals, as when we eat bread made from the 

 reserve stores of the wheat seed, or beans, peas or nuts, 

 which are also seeds with large stores of food. 



Thus, the food of the organism <?5 a whole is very 



