94 PLANT ANATOMY. 
form the so-called infusorial earth, are of the greatest import- 
ance in practical life. 
THE CELL-WALL AND CONTENTS OF THE CELL. 
I. Contents of the Cell. 
The cell consists of the cell-wall, and the contents of the cell. 
The most essential constituent of the cell while exercising the 
functions of life, is the protoplasm (plasma’). This repre- 
sents a turbid, semi-liquid mass, which completely fills the inner 
portion (lumen) of cells located at the growing point, or which 
are otherwise in a state of active development. At a later 
period, there appear in the protoplasm cavities or vacuoles.* 
The latter, filled with colorless cell-sap, constantly continue to 
enlarge as the cell becomes older, coalesce with each other, and 
finally, while the protoplasm gradually contracts toward the 
wall of the cell (protoplasm-sac, primordial utricle, cell-sac), 
form a large, central cavity, filled with cell-sap (Fig. 27 p). If 
the cell has ceased to grow, the protoplasm will also have disap- 
peared, with the exception of a delicate film attached to the 
membrane. The protoplasm takes the most active part in all 
formative processes in the cell, and is the most important sub- 
stance in the cell; the formation of the cell-wall proceeds from 
it, and to it the other constituents of the cell, for the most part, 
owe their origin. 
The cells increase in number only by division. A cell (mother- 
cell) becomes divided by a septum (which is mostly median) 
into two daughter-cells (Fig. 28). And it is considered asa law 
that with the division of the cell (of the protoplasm) a division of 
the nucleus (see below and Fig. 28) is also always associated. In 
the case of reproductive cells, a rounding of their form is also 
generally associated with their division. 
The protoplasm, which always possesses a semi-liquid, and 
(with the exception of the outermost and innermost layer, hya- 
loplasm) a granular character (microsomes), is a body of compli- 
' p@roy the first, and 2la6ua organization or form. 
* Vacuum, empty. 
