PLANTS AND ANIMALS DISTINGUISHED 219 



(1) Origin. Both branches of the tree of life start 

 alike : the lowest of plants and animals consist of a 

 single cell. In fact, the cycle of life in all living beings 

 begins in a small, round particle of matter, a cell in 

 the higher plants called an ovule, in the higher animals 

 an ovum. This cell consists mainly of a semifluid sub- 

 stance called protoplasm. In the very simplest forms 

 the protoplasm is not inclosed by a membrane or cell 

 wall. In most plants the cell wall is present, and con- 

 sists of cellulose, a substance akin to starch; in animals, 

 with few exceptions, the wall is a pellicle of firmer pro- 

 toplasm, i.e., albuminous. 



(2) Composition. Modern research has broken down 

 the partition between plants and animals, so far as 

 chemical nature is concerned. The vegetable fabric 

 and secretions may be ternary or binary compounds ; 

 but the essential living parts of plants, as of animals, 

 are quaternary, consisting of four elements, carbon, 

 hydrogen, oxygen, and nitrogen. Cellulose (woody 

 fiber), starch, and chlorophyl (green coloring matter) 

 are eminently vegetable products, but not distinctive ; 

 for cellulose is wanting in some plants, as some fungi, 

 and present in some animals, as tunicates ; starch, under 

 the name of glycogen, is found in the liver and brains 

 of mammals, and chlorophyl gives color to the fresh- 

 water polyp. Still, it holds good, generally, that plants 

 consist mainly of cellulose, dextrin, and starch ; while 

 animals are mainly made up of albumen, fibrin, and 

 gelatin ; that nitrogen is more abundant in animal tis- 

 sues, while in plants carbon is predominant. 



(3) Form. No outline can be drawn which shall be 

 common to all animals or all plants. The lowest mem- 

 bers of each group have no fixed shape. The spores 

 of Confervae can hardly be distinguished from animal- 

 cules ; the compound and fixed animals, sea mat and 



