PROTOPLASM 113 



Although the microscope was invented many centuries ago, 

 it had to undergo much improvement before the cellular structure 

 of organisms could be recognized. Robert Hooke (1635-1703), 

 an EngHsh inventor, so improved the microscope that he could 

 recognize the cellular structure of plant tissues. In applying 

 his improved microscope to the study of thin sections of charcoal 

 and cork, he saw the cell cavities and the cell walls. He de- 

 scribed cork as being composed of numerous cavities separated 

 by thin walls. He thought the structure of cork was comparable 

 to the compartment-like structure of honeycomb, and therefore 

 apphed the term cells to the compartments of cork. Since the 

 protoplasm in the cells of cork is dead and dried up, Hooke did 

 not see the important substance of cells, the term cell, as he used 

 it, meaning nothing more than a cavity with its enclosing walls. 

 In 1835 Dujardin, a French naturahst, recognized the Hving 

 substance in the cells of animals. He called it sarcode. About 

 eleven years later Von Mohl, a German biologist, recognized the 

 living substance in plant cells. He called it protoplasm. Among 

 the many contributors to our knowledge of cells during the middle 

 part of last century Schleiden, Schwann, Nageli, and Max Schulze 

 should be mentioned. By 1870 the sarcode of animals and 

 vegetable protoplasm were found to be identical and the term 

 protoplasm was applied to both. It was also recognized that 

 protoplasm and not the cell wall is the important substance of 

 ceUs. During this period the nucleus was discovered and cell- 

 division observed. Also that protoplasm is the only living sub- 

 stance of plants and animals and that it constructs the other 

 plant and animal structures were now beginning to be recognized. 

 Protoplasm. — The protoplasm, as already noted, is the Uving 

 substance of plants and animals. The protoplasm of an indi- 

 vidual cell is often called a protoplast. Protoplasm is a fluid 

 substance which varies much in its consistency, sometimes being 

 a thin viscous fluid Uke the white of an egg, and sometimes being 

 more dense and compactly organized. Chemical analyses show 

 that protoplasm has the composition of protein, although such 

 analyses necessarily kill the protoplasm and consequently do not 

 give us a true knowledge of the protoplasm as it is while hving. 

 Although the protoplasm of higher plants usually exhibits no 

 motion except when dividing, there are cases, however, as in the 

 hairs of the Pumpkin and Wandering Jew, where the protoplasm, 



