THE CELL AND THE CELL THEORY 55 



they termed variously gum, mucilage, and sarcode. The real 

 nature of this substance within the cell wall was not, however, 

 positively known until about the middle of the last century, 

 when its true significance as the essential living substance of 

 the cell and of the organism was recognized. The name proto- 

 plasm, which it now bears, was first given to this living sub- 

 stance in animal cells by Purkinje (1839-1840) and in plant 

 cells by Von Mohl (1846). Von Mohl's conception of the 

 nature of the living substance of plant cells, including the 

 nucleus, is embodied in the following quotation, taken from 

 his work on " The Vegetable Cell." 



In the center of the young cell, with rare exceptions, lies the 

 so-called nucleus cellulae of Robert Brown. It is usually of very 

 considerable size in proportion to the magnitude of the young 

 cell, so that in particular cases for example, in the cells of jointed 

 hairs it almost fills the cavity. The remainder of the cell is more 

 or less densely filled with an opaque, viscid fluid of a white color, 

 having granules mingled in it, which fluid I call protoplasm. 



Von Mohl and his contemporaries, Schleiden and Nageli, 

 not only clarified the current conceptions of the nature of the 

 cell and of protoplasm but also established the fact of cell 

 differentiation, or change in the form and structure of cells to 

 serve different functions in the plant. They discovered that the 

 stems, roots, and leaves of plants are composed of a great 

 variety of cells, varying in form, structure, and function, which 

 enable the plant organism to do its work more effectively than 

 it otherwise could. 



The cell theory. The idea that all living plants and animals, 

 however diverse in their nature, are composed of similar cell 

 units was first published by Schleiden, a botanist, and Schwann, 

 a zoologist, in 1839. Later investigations have not only cor- 

 roborated the truth of this generalization but have greatly 

 extended its meaning and emphasized its importance. It has 

 been found also not only that animals and plants are similar 

 in general cellular structure but that the living substance is 

 not essentially different in the two kingdoms of life. The 



