PLANTS AND ANIMALS. 21 



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- 

 grown parts of plants exhibit movements. Moreover, the 

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

 tentacles of the sundew, 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 various modifications 

 of cells. In short, all organisims have a cellular structure. 

 This general conclusion is part of the Cell Theory or Cell 

 Doctrine. 



(c) Resemblance in development. — When we trace the 

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

 arises from a unit element 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- 

 tinued division, arrangement, and modification of cells. 

 Thus plants and animals resemble one another in their 

 essential functions, in their cellular structure, and in their 

 manner of development. 



Contrasts. — But while there is no absolute distinction 

 between plants and animals, they represent divergent 

 branches of a V-shaped tree of life. It is easy to distinguish 

 extremes like bird and daisy, less easy to contrast sponge 

 and mushroom, well-nigh impossible to decide whether 

 some very simple forms, which Haeckel called "protists," 

 have a bias towards plants or towards animals. We cannot 

 do more than state average distinctions. The food which 

 most plants absorb is cruder or chemically simpler than that 



