532 MANUAL OF ELEMENTAR Y ZOOLOGY 



fact that green plants cannot live 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 a true respiration like that of animals, 

 although this is obscured by the reverse process taking 

 place to a greater extent during daylight, (iii) While the 

 food of animals consists of complex organic substances, 

 usually in the state of a solid or the viscous liquid proto- 

 plasm, 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. We have seen, 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 in various cases, from the living or dead bodies of 

 other organisms. 



3. From the mode of nutrition of plants there follows the 

 third character which we have marked in them. In the 

 great majority of animals food must either be sought by 

 locomotion or at least seized by other active movements, 

 as it is, for instance, in Obelia or Vorticella. In plants, on 

 the other hand, not only is this necessity absent, but, since 

 it is desirable that they should expose as great a surface 

 as possible to air and water for absorption — as they do, 

 for example, in leaves and roots — the shape of their bodies 

 is necessarily such as to be an actual hindrance to motion. 

 Thus in most plants active motion is restricted or absent, and 

 muscular and nervous tissues are not found in plant bodies. 



4. The necessity for surface leads to a fourth character 

 in plants. An extensive surface needs strong support. In 

 correspondence with this need we find in plants a massive 

 skeleton which forms a strong wall to each cell, so that the 

 protoplasm is upheld by an intricate framework of com- 

 partments whose walls are thickest in the most woody 

 parts of the body. Owing, no doubt, to the ample supply 

 of starch at the command of the plant, this skeleton consists 

 of a modified form of starch known as cellulose. Among 



