PLANTS AND ANIMALS 41 



subsciences which in general correspond. Thus one can speak of plant 

 morphology and animal morphology, of plant ecology and animal ecology, 

 of plant physiology and animal physiology, and the same is true of 

 taxonomy, pathology, embryology, and so on. 



68. Differences between Plants and Animals. — The higher plants 

 and higher animals, as has already been stated, present distinctions which 

 are sufficient in all cases to enable us to assign a hving thing to either one 

 category or the other. Among these distinctions are the following: 



1. Movement. — Broadly speaking, the higher plants lack the power of 

 movement and in all cases are without the power of locomotion. On the 

 other hand, almost without exception, animals are possessed of both. 



2. Manner of Growth. — In a general way it may be said that the 

 plant grows by the addition of parts externally, such as the addition of 

 leaves and twigs. There is also evidence of internal growth, as is seen in 

 the gradually expanding trunk and constantly thickening branches, 

 the new wood being added just underneath the bark. In animals, on the 

 other hand, few parts are seen to be added externally, though some ani- 

 mals show at times a gradual increase in size of wings, horns, and other 

 visible parts ; growth is mostly internal and the body simply increases in size. 



3. Cells. — Plant cells usually possess a distinct cell wall, composed of 

 cellulose, which gives to the cell rigidity of form and to which is due the 

 immobility of the plant body. Animal cells, on the contrary, often 

 possess no wall of any kind, and the walls, when they are present, are 

 generally thin and permit the cell to change shape. This fact contributes 

 to the power of movement and of locomotion possessed by animals. 



4. Food Securing and Metabolism. — There are minor differences 

 between plants and animals connected with the metabolism of the two, 

 but the steps are essentially the same (Fig. 10). However, the oxidation 

 changes in the cells which are included in this text under the term dis- 

 similation are by botanists termed respiration. Plants, in addition to 

 carrying on the same type of metabolism as has been described for 

 animals, have the power of manufacturing their own complex foods. By 

 virtue of their possession of plastids and of their ability to utilize the 

 energy of the sun's rays they can take simple substances from the earth 

 and air and out of them synthesize complex substances such as proteins, 

 fats, and carbohydrates. These processes are known as photosynthesis. 

 After producing these substances plants make use of them in the same 

 fashion as do animals, but animals being incapable of manufacturing 

 such foods have to get them from plants or by eating other animals. 



Since plants take these simple substances in gaseous or liquid form 

 there is no soHd waste left as a result of plant metabolism, and conse- 

 quently egestion does not occur. Also, since plants build up proteins, 

 which they add to their substance, and make immediate use of any 

 nitrogenous matter liberated in protoplasmic activity, they do not 



