DIFFERENCES BETWEEN ANIMALS AND PLANTS 225 



hydrates, fats, and proteids; combines them with oxygen, and, 

 as a result, produces as waste products carbon dioxid, water, 

 and urea. The foods are broken to pieces, and the energy 

 thus liberated is utilized; see Chapter XV. In plants the proc- 

 ess is primarily constructive, but there is in plant life both 

 a constructive and a destructive metabolism. By the former 

 the plant uses carbon dioxid, water, and nitrates, which 

 are combined in the plant to form organic substances, like 

 starches, proteids, etc., and in the combination solar energy 

 is stored away. As an excretion, there are produced oxygen 

 and water. The destructive process of plants is essentially like 

 that of animals: the compounds built up by the first process 

 are destroyed by the second. The total amount of construction 

 in green plants is greater than the amount of destruction, 

 and therefore the green plants store away organic products 

 which may subsequently be utilized by plant life. 



4. Respiration. Animals usually have lungs or gills filled 

 with blood ; they always absorb oxygen, and eliminate the car- 

 bon dioxid. In plants the respiration is carried on through the 

 stomata of the leaves; when carrying on photosynthesis, plants 

 absorb carbon dioxid and eliminate oxygen; when not carry- 

 ing on photosynthesis, the gas absorbed is oxygen and the gas 

 liberated is carbon dioxid. 



5. Excretion. In animals carbon dioxid is excreted from 

 the lungs, water from the skin and kidneys, and urea from 

 the kidneys. In plants there is no well-developed excretory 

 system, although gases are excreted through the stomata, 

 and certain other substances may pass out through the bark 

 or through the roots into the soil. 



6. Motion. The muscles of animals develop a high degree 

 of motion. In plants motion is very rarely developed, although 

 it is not wholly lacking, some plants being well supplied with 

 motile power. They do not, however, have muscles; and when 

 they have motion, they use other forms of mechanism. 



7. Support. Animals usually have a skeleton of shell or 



