THE CELL. 



79 



free and when confined by a cell-wall. Swarm-spores (Fig. 

 169), which consist of a mass of protoplasm des- 

 titute of a covering, swim through water by means 

 of their cilia, or hair-like protoplasmic elongations. 

 The slime moulds consist, in their vegetative or 

 growing stage, of naked protoplasm, and this is 

 able to creep slowly about over rotten wood in 

 damp forests, where they usually grow. Diatoms 

 (Fig. 182), which are one-celled plants, move 

 freely about in the water, though in exactly what ^^^ 

 manner they accomplish the movement is as yet not satis- 

 factorily determined. The protoplasm confined in cell- 

 walls exhibit two kinds of movement, called circulation 

 and rotation. 



I02. The circulation of the protoplasm (Fig. 183) 

 takes place in cells having a large sap-cavity, the proto- 

 plasm existing as a parietal layer, connected by strings 

 and bands with a more or less central mass ; it consists of a 

 movement (shown by the granules) in streams, mainly to and 

 from the nucleus. The currents may be in opposite direc- 

 tions, though side by side, and often 

 contiguous ; they may gradually cease 

 and then begin again in an opposite 

 direction without apparent cause. The 

 movement called rotation (Fig. 18-t) 

 difiers from this, in that the whole 

 mass of the protoplasm rotates or 

 moves as a broad stream around the 

 cell-wall, passing up one side and 

 ' 183 down the other, carrying the granules 



Fig, 182. A Diatom, Navicula viridis. Fig. 183. Circulation of Protoplasm in a 

 cell ; the arrows indicate the direction of the currents. 



