28 



ELEMENTS OF PHYSIOLOGY 



V 



appears an interlacing series of beads and lines (Fig. 

 26). But there must be some test for identifying pro- 

 toplasm, or we should never be able to tell organic from 

 inorganic matter ; animal and plant substance from 

 mineral. The only distinguishing feature yet known 

 about protoplasm is that there are always found in it 

 certain complex albuminous substances that are never 

 found in inorganic bodies. The white of an egg, also 

 called albumen, is such a substance. 

 The power that the cells possess of per- 

 forming the processes of life depends 

 upon the presence of these albuminous 

 substances. The albumin, or nitroge- 

 nous substance, in protoplasm makes it, 

 therefore, the life substance, and dis- 

 tinguishes it from the mineral world. 

 Nothing in the world besides animals 

 and plants possesses the power of 

 growth by converting other material 

 into its own kind of substance. 



43. Difference between Animals and 



'"Water 



FIG. 24. Diagram. 



The plant taking in carbonic 



acid gas, called also car- Plants. Can you always tell an animal 



bon dioxid, the product of ^ n ^ ^ ? 



fire and of animal life. By 



from a plant ? You say an animal can 



means of the sun's energy mQve f rom p j ace to p l ace ; the TCply is, 

 the leaf builds the carbon 



into living forms and gives no, a sponge and many other animals 



back the oxygen to the air. . . , 



cannot. You say an animal has nerves 

 and feeling, but some simple animals are without nerves ; 

 on the other hand, some plants respond to a stimulus or 

 irritation, although they never develop special organs 

 for transmitting the stimulus. It is easy to distinguish the 

 higher animals from plants ; but in regard to many of the 

 lowest forms of single-celled organisms, the most learned 

 biologists find it impossible to say whether they are plants 

 or animals. The clearest difference between animals and 

 plants is, that plants get their food directly from mineral 



