PHYSICAL CHARACTERISTICS 15 



fifty years later, through the labors of a great many men, 

 that it was gradually realized that the cells in the living 

 parts of plants and animals are always filled with a jelly- 

 like substance, and that this is really the living substance. 



In 1844 Carl Naegeli first enunciated what we recog- 

 nize to-day to be the correct theory of growth: That 

 every living cell is derived from another living cell by 

 division. The full force of this statement can only be 

 appreciated when it is realized that the fanciful theories 

 in favor at that time supposed that cells might be pro- 

 duced in something like the same way that the pores in 

 a loaf of bread are formed by the expanding gases. Thus 

 it is seen that this was a very important discovery. It 

 was not long until it was recognized as a correct state- 

 ment of fact, both for plants and animals. At about the 

 same time it began to be realized that the contents of 

 the cell are of importance, and in 1846 Hugo von Mohl 

 recognized this fact by giving the name protoplasm to 

 the slimy cell contents. Within the next twenty years 

 it became an established fact that the important part 

 of the cell is the protoplasm and that it is the seat of all 

 life activities. 



Physical Characteristics of Protoplasm. — It 

 might be supposed that a substance so important to all 

 living things as protoplasm would be distinguished by 

 easily recognizable and striking characteristics. This, 

 however, is not true. Protoplasm from different sources 

 appears very much the same. It is a jelly-like substance, 

 that is to say, it is neither a solid nor a liquid, being in 

 consistency somewhat like the white of an egg. It is 

 either colorless or slightly yellow in appearance. It is 

 not sufficiently fluid to run like water, nor is it sufficiently 

 rigid to stand up like a solid jelly. Its own weight is 

 sufficient to cause it to spread out on the surface of a 

 glass slide. It can be pulled to one side like rubber and 

 will retract to its original form when released, showing 

 that it has a certain degree of elasticity. Some kinds of 



