LIVING THINGS 215 



made of the same materials, differ considerably in composi- 

 tion. The animals have characteristically nitrogenous tissues, 

 and the plants carbonaceous tissues. 



181. Digestion. The food stores of plants are usually in 

 insoluble form, and have to be made soluble, or digested, before 

 they can be built up into living tissues. Digestion is accom- 

 plished by the aid of certain ferments called enzymes, and re- 

 quires the presence of a certain amount of water for their 

 activities. The process is comparatively simple in plants, but 

 in all but the simplest animals there are special organs for 

 containing the food during the digestive process and various 

 glands for secreting the enzymes and other digestive fluids 

 needed. The digested food is carried in solution to the point 

 in the body where it is used. In plants, the solution is com- 

 monly called sap; in animals, it is the blood. 



182. Respiration. One of our commonest sources of energy 

 is the union of oxygen with carbon. In living things this is 

 practically the only source of energy. Both animals and 

 plants respire, taking in oxygen for the purpose, and giving 

 out carbon dioxide. The real respiration occurs in the cells, 

 but in animals there are usually organs for rapidly carrying 

 oxygen to them. Plants, being less active than animals, re- 

 spire more slowly, but the process is the same in all. From 

 the fact that in photosynthesis plants take up carbon and give 

 off oxygen, it is often assumed that the process of respiration 

 or breathing in plants is exactly the opposite of that in animals, 

 but this is a mistake. Plants respire like animals, but have 

 in addition, the capacity for photosynthesis in which they 

 take in carbon dioxide and give off oxygen. It is interesting 

 to note that the formation and use of food is somewhat analo- 

 gous to what goes on in the storage battery in that the energy 

 from the sun, like the electric current, is made to produce 

 certain changes in matter, and this energy is obtained again 

 when the changes in matter are reversed. 



