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28 THE HISTORY OF CELLS 



the cells which he called the nucleus ; and later he found a like 

 structure in the cells of many other plants. He wrote the following 

 account of his discovery : " In each cell of a great part of the 

 orchid family, a single circular area, generally somewhat more 

 opaque than the membrane of the cell, is observable. The 

 nucleus of the cell is equally manifest in many other fam- 

 ilies. ..." 



Theodore Schwann (1810-1882) found, as a result of careful 

 work, that all the animals he investigated were made of cells. 

 At the same time a friend of his, Matthias Schleiden, discovered 

 that all of the plants he observed were made of cells. In the 

 cells they studied, they both noted the nucleus that Brown had 

 first discovered. They then made the supposition that all living 

 things are made of cells. This became known as the cell theory, 

 and this theory, having since been checked and rechecked by 

 many scientists, and more facts added, is now accepted as a fact. 



Protoplasm was discovered by a French naturalist, Felix Dujar- 

 din, in 1835, and named by Purkinje in 1840. But the scientist 

 who made the name protoplasm best known was Hugo von Mohl 

 (1805-1872). He was born in Stuttgart, Germany. He was 

 graduated in medicine from Munich, became a professor of phys- 

 iology at Berne, and later a professor of botany at Tubingen. He 

 observed and analyzed the cell contents and described the move- 

 ment in the cell of certain small bodies which later were termed 

 chloroplasts. These were later shown to be the structures that 

 contain green coloring matter. He brought into general use the 

 term protoplasm and the fact that it is a living, streaming, grow- 

 ing, dividing substance. 



Problem. How do the cells of Elodca appear when viewed through the 

 microscope ? 



Examine the appearance of a single leaf of the Elodea. Mount it in 

 water and heat by holding in the palm of the hand for a few minutes. The 

 heating will activate the protoplasm. Focus with low, then high power. 



