154 THE NATURE AND WORK OF PLANTS 



different things than single organs of the horse do. The 

 tree does not have so many separate organs, each for a dis- 

 tinct purpose, as the horse has. It has no eyes, no ears, 

 no teeth. Its structure is simpler than that of the horse. 



When, however, we go down the scale of life and com- 

 pare the simpler plants with the simpler animals, it grows 

 harder and harder to tell plant from animal. The sponge 

 that may be seen in the ocean, and even in some farm 

 brooks, cannot move from place to place although it is an 

 animal. On the other hand, certain low plants, like the 

 pond scums that may grow beside the sponge, have much 

 freedom of motion. 



The Difficulty becomes still greater when we come to 

 bacteria. For more than a century after they were dis- 

 covered, they were taken to be little animals. Now, how- 

 ever, they are regarded as plants. 



103. Protoplasm. The white of an egg is a substance 

 familiar to all of us. It is jelly-like. It lacks stable 

 form ; and, if pressed between the fingers, no grit is felt 

 in it. It is thicker than water, of which it is mostly com- 

 posed. If heated, as is well known, it coagulates; that 

 is, it becomes white and opaque. Certain chemicals also, 

 like alcohol, will coagulate it. A lump of sugar can be 

 kept, at ordinary temperatures, for any length of time, 

 without decomposing. But the white of an egg, unless 

 carefully preserved, soon spoils and gives off an offensive 

 odor. 



This white-of-egg substance we call albumen, or protein. 

 Now protein is a large part of protoplasm, about which we 

 wish to learn. Chemists tell us that protoplasm is ex- 

 ceedingly complex. Huxley, a great English scientist, 

 called it "the physical basis of life." It is present in all 

 the growing parts of plants and animals, and, under a 

 powerful microscope, it has exactly the same visible char- 



