118 PLANT PHYSIOLOGY 
the light is dim, they are carried to the top or bottom 
of the cell, where they will get the strongest light broad- 
side. If the light is too strong, they are carried to the — 
sides of the cell, where the light will only strike them 
edgewise. 
174. The locomotion of cells is accomplished mostly 
by the lashing movements of slender cytoplasmic pro- 
jections from the surface of the naked cell. If few in 
number and long, they are usually called flagella. If 
numerous and rather short, they are called cilia. When 
single or few, they are usually attached at the anterior 
end of the cell. A few plant cells 
move by amoeboid motion, i.e. 
send out processes or lobes into 
which the whole protoplasm flows. 
- The cells of diatoms (Bacillario- 
ideae) are provided with cell walls 
es wa ae ae of cellulose so filled with silica as 
(Ulothrix, Pleurocladia, to be non-elastic and_ brittle. 
Marchantia, Struthiopteris, * 
Zamia). In some diatoms the protoplasm 
comes to the surface through a 
longitudinal slit, the raphe, and its longitudinal motion 
in this slit is probably the cause of the motion of the cell. 
Finally, must be mentioned the motion of some diatoms 
as well as desmids, and some of the blue-green algae 
(e.g. Oscillatoria) which is ascribed to the secretion of a 
slime through the cell wall. The bending of the 
Oscillatoria filaments, however, may be due to proto- 
plasmic contraction. 
175. All of these movements are dependent onan ample 
supply of oxygen, and cease very quickly in its absence. 
The usual cardinal points of temperature can be found 
for these as well as for other functions of the cell. Ap- 
parently the movements within the cell are of use in — 
