THE CELL 15 
The walls were considered the important feature, and the 
term cell meant the space enclosed by the wall. All this 
was very natural, for botanists had quite generally, up 
to this time, devoted their best energies to studying the 
form and structure of plants, paying relatively little at- 
tention to their life functions, or physiology. Gradually, 
however, it came to be recognized that the really impor- 
tant part was the substance that filled the little compart- 
ments in all living tissues. 1 It finally came to be under- 
stood that this is the only living substance in plants 
(and in animals as well), and that the cell-walls, and in 
fact the entire organism, are built up by the activity of 
this remarkable substance. It was first called by several 
different names, but Hugo Von Mohl, a noted German 
botanist, called it protoplasm,^ considering it as the first 
organic substance formed from the inorganic materials 
taken in by the plant. This name was generally adopted 
by both botanists and zoologists. 
22. The Cell-theory. The idea that all living things 
are composed of cells, that the cell is the unit of plant and 
animal structure, and that the essential thing about the 
cell is the protoplasm, was elaborated by Schleiden (1838) 
(for plants) and by Schwann (1839) (for animals), and 
was accepted as generally correct by all students of plants 
and animals. This doctrine became known as the cell- 
theory of Schleiden and Schwann. The term cell is now 
used in biology chiefly to designate the protoplasm comprised 
within the cell-wall. A cell, then, is not a compartment 
containing something, but is a structural unit of living 
1 An intimately connected layer, group, or body of similar cells, all 
having like functions, is a tissue. 
* Greek protos (first) + plasma (thing formed). 
