104 A TEXT-BOOK OF BOTANY 
In the great majority of cells there is a single nucleus, and 
all about it, filling the general cavity within the cell-wall, 
is a mass of much less dense protoplasm, known as cytoplasm. 
The cytoplasm seems to form the general background or 
matrix of the cell, and the nucleus lies embedded within it. 
Another protoplasmic organ of 
the cell is the plastid. Plastids 
are relatively compact bodies, 
and variable in form and num- 
ber. The most common kind of 
plastid is the one that contains 
chlorophyll, and hence is known 
as the chloroplastid or chloroplast. 
An ordinary ceil of an alga, 
therefore, consists of a cell-wall, 
within which the protoplasm is 
organized into cytoplasm, nu- 
cleus, and chloroplasts. With 
= eee ie es ee proper staining the nucleus and 
and cytoplasm (c). the chloroplasts of Pleurococcus 
can be seen; but these structures 
may be seen more distinctly and with much less trouble 
in the cells of a moss leaf (Fig. 96). 
The cell-wall is elastic, so that the cell can be compressed 
or inflated. The single cell of Pleurococcus, unless pressed 
upon by neighboring cells, retains a spherical form as long 
as it is alive, a fact which shows that there is constant and 
uniform pressure on the wall from within the cell. It is 
found that this pressure is due to the absorption of water 
in sufficient amount to stretch the wall, this distended con- 
dition of the cell being called turgor, a name indicating 
that the cell is turgid. Pleurococcus retains its spherical 
form, therefore, because it is turgid; and the bulging of 
free walls of Oscillatoria ($ 60) is due to the turgor of 
the cells. 
