PLANTS AND ANIMALS. 17 



in their analysis, for they are endeavouring to discover the physical and 

 chemical changes associated with the living stuff or protoplasm itself. 

 These are obviously at the foundation of the whole matter. 



Plants and Animals. 



Before we give a sketch of the chief functions in a higher 

 animal, let us briefly consider the resemblances and differ- 

 ences between plants and animals. 



(a.) Resemblance in Function. The life of plants is 

 essentially like that of animals, as has been recognised since 

 Claude Bernard wrote his famous book, Phenomenes de la 

 vie communs aux animaux et aux vegetaux. The beech 

 tree feeds and grows, digests and breathes, as really as does 

 the squirrel on its branches. In regard to none of the main 

 functions is there any essential difference. Many simple 

 plants swim about actively ; young shoots and roots also 

 move; and there are many cases in which even the full- I 

 grown parts of plants exhibit movements. Moreover, the ' 

 tendrils of climbers, the leaves of the sensitive plant, the 

 tentacles of the sun-dew, the stamens of the rock rose, the 

 stigma of the musk, are but a few instances of the numerous 

 plant structures which exhibit marked sensitiveness. 



(b.} Resemblance in Structure. The simplest plants (Pro- 

 tophyta) like the simplest animals (Protozoa) are single 

 cells ; the higher plants (Metaphyta) and higher animals 

 (Metazoa) are built up of cells and of various modifications A 

 of cells. ' In short, all organisms have a cellular structure, 1 

 This general conclusion is known as the Cell Theory or 

 Cell Doctrine (see p. 41). 



(c.} Resemblance in Development. When we trace the 

 beech tree back to the beginning of its life, we find that it 

 arises from a uniLjelement or egg cell, which is fertilised by 

 intimate union with a male element derived from the pollen- 

 grain. When we trace the squirrel back to the beginning 

 of its life, we find that it also arises from a unit. element or 

 egg cell, which is fertilised by intimate union with a male 

 cell or spermatozoon. Thus all the many celled plants and 

 animals begin as fertilised egg cells, except in cases of 

 virgin birth (parthenogenesis) or of asexual reproduction. 

 From the egg cell, which divides and redivides after fertilisa- 

 tion, the body of the plant or animal is built up by con- 



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