THE MOVEMENTS OF PLANTS. 



167 



The latter are usually due to the protrusion of portions of the 

 protoplasm through slits in the wall, as in some diatoms- 

 (fig. 11). The filaments of .the water slimes bend from side 

 to side, and so creep over wet surfaces very slowly (fig. 7). 

 Bacteria (fig. 9) and some diatoms move by means of cilia. 



The direction of all these movements may be so controlled 

 by stimuli that the organisms move toward or 

 away from the source of stimulus. Thus, ciliated 

 spores of algae (fig. 109) swimming in a dish of 

 water, will gather next the lighter side. 



238. Streaming and rotation. — In multicel- 

 lular organs it is common to find the protoplasm 

 within each active cell moving about from point 

 to point within the cell. The protoplasm is 

 filled with numerous large vacuoles, so that it 

 forms a next layer the wall, with threads of 

 ribbons extending across it (fig. 123). When 

 currents start along the wall and through the 1 .y i| 

 strands, the motion is designated as the stream- 

 ing of the protoplasm. These currents along 

 any particular portion of the protoplasm may 

 run side by side and in opposite directions. 



When the protoplasm surrounds a single 

 large vacuole (fig. 117, C), the whole mass 



Fig. 123.— a 



single 



cell from a hair of 



,,.,,. . _ . , Ch el idoniitjft. 



may rotate, usually in the direction of its long The arrows show 



„,, . . , . . . the direction of 



axis. Ine portion immediately in contact movement of the 



protoplasm in the 

 peripheral layer 

 and in the bands 

 which separate the 

 vacuoles. «. the 

 nucleus, with nu- 

 cleolus. Highly- 

 magnified. — After' 

 Dippel. 



with the wall is motionless, and there must 

 necessarily be a strip between the half moving 

 up and the half moving down the cell, which 

 is also quiet. Such movements are called 

 rotation of the protoplasm. It is not known 

 whether either streaming or rotation has any immediate re- 

 lation to the well-being of the cell. 



In addition to the mass movements of the protoplasm, the 



