ANIMALS AND PLANTS! DIFFERENCES 21 



of the plant. This function is the obtaining of carbon from carbon 

 dioxide by means of the energy of the sun's rays, and the use of 

 it in the manufacture of complex organic substances. Absorbing 

 certain rays of light, the chlorophyll enables the protoplasm to 

 use the energy of the rays to reduce (in the chemical sense) 

 molecules of water and carbon dioxide so that the products 

 can combine to form carbohydrates. This process is known as 

 photosynthesis, and is accompanied by the liberation of oxygen. 

 This can easily be shown in the case of water plants, from whose 

 leaves in sunlight a stream of fine bubbles of oxygen may be 

 seen to ascend. The carbohydrates are used for the formation 

 of various organic substances present in the protoplasm of plants, 

 and in particular of proteins. The nitrogen, sulphur, and phos- 

 phorus for this purpose are obtained by the plants as salts in 

 solution in the water which is taken in by their roots, or sometimes, 

 as in seaweeds, by the whole surface of the body. From this 

 peculiarity of nutrition arise several other features peculiar 

 to the life of plants, (i) We have here the reason for the well- 

 known fact that green plants cannot live indefinitely in the dark, 

 (ii) While animals, as we have seen, are always taking in oxygen 

 and giving out carbon dioxide, green plants in the light are 

 continually taking in carbon dioxide and giving out oxygen. 

 Yet it must be remembered that the protoplasm of plants 

 undergoes continually respiration like that of animals although 

 this is obscured by the reverse process taking place to a greater 

 extent during daylight, and that some animal tissues can assimi- 

 late carbon dioxide, though not by photosynthesis, (iii) Though 

 the material included in the protoplasm is similar in the two kinds 

 of organisms, plants manufacture its organic components from 

 simple substances, whereas animals obtain them from other 

 organisms or their products. Therefore, while the food of animals 

 consists of complex organic substances, usually in the state of 

 a solid or liquid protoplasm, and has to be swallowed through an 

 opening, the materials taken in by green plants are simple 

 inorganic substances which can be absorbed as gases or liquids 

 through the surface of the body. It must be noticed, however, 

 that plants which have no chlorophyll, such as Fungi, and some 

 animals which live as parasites or in decaying matter, absorb 

 their nourishment through the surface of the body, but take it 

 in the form of organic substances, more or less complex, from 

 the living or dead bodies of other organisms. 



