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 

 of cellulose so filled with silica as 



Fig. 52. — Flagellate cells, , i , • i i • j. . i 



(uiothrix, pieurociadia, to be non-elastic and brittle. 



Marchantia, Struthiopteris, t , . , , . . -, 



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 on an 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 



