, 
; 
ANATOMY OF THE CELL. 25 
two kinds; first, that which is hmited by the cell wall, and 
which produces no change in the position of the cell or plant ; 
second, that by which the plant is enabled to move from place 
to place. In certain cells currents of protoplasm have been 
discovered varying in their action according to the distribution 
of vacuoles and consequent arrangement of the protoplasmic 
substance. Where there are numerous vacuoles extending 
more or less evenly through the cavity, the current is irregular, 
following the windings of the protoplasmic meshes. In such 
cases it is called circulation. An example of it is found in 
stamen hairs of Tradescantia. In other cells where the greater 
part of the protoplasm is pressed back upon the cell wall, a 
regular current is seen passing entirely around the cell, carry- 
ing with it some of the structures before referred to. This 
action is known as rotation. Example, Nitella. 
The second kind of motion occurs in ease of small, unicellu- 
lar plants with or without membrane, and corresponds. exactly 
to that in animals of like low organization. Among certain 
bacteria, and also swarm spores of various algae and fungi, cilia 
occur, by whose alternate contraction and relaxation the cell is 
enabled to move about from place to place. There are also 
small multicellular plants having the power of motion from 
place to place, the mechanism of which is supposed to be 
similar to that producing certain motions in the members of 
higher plants, where the individual as a whole occupies a fixed 
position. 
2. Cell Nucleus. 
This appears as a roundish oval or lens-shaped body sharply 
set off from the surrounding medium ; it consists of a clear sub- 
stance containing numerous small granules and one or more 
larger grains called nucleoli. . It is bounded by a dense portion 
of the clear ground substance. Cells ordinarily have but one 
nucleus, but in many of the lower plants several nuclei occur 
in the same cell. 
