358 



PLANT PHYSIOLOGY 



only in the layer of protoplasm lining the cell wall but in the 

 various strands of cytoplasm which cross the vacuole, while in 

 rotation the motion occurs only in the cytoplasmic layer lining 

 the wall. Circulation is very common in land plants and may be 

 easily observed in the staminal hairs of the spiderwort (Trades- 

 cantia), while rotation is more often seen in water plants such as 

 Nitella and Elodea. The movements may be normal or they may 

 be due to excitation produced by abnormal conditions. Rusk 

 (1920) and Seifriz (1922) have reported that zinc, lithium, and 

 other salts cause very rapid circulation and rotation of the proto- 

 plasm, which fact may be used to advantage for demonstration 

 work. 



In very large cells or ccenocytes, as found in the slime molds 

 (Myxomycetes) , Rhizopus, and Vaucheria, the protoplasm may 

 move down through the center of the cell and back along the 

 edges much as a current moves in a closed pipe. Such movement, 

 closely related to rotation, is called streaming and may help to 

 distribute the food and other materials within the cell. 



Certain portions of the cell (nucleus or plastids) may orient 

 themselves inside of the cell in response to external stimuli. Thus 



i 



i 



Fig. 26. — Orientation of chloroplasts in the light. A shows the position of 

 the plastids in weak light. As the light becomes stronger the plastids of the 

 palisade cells move toward the end away from the light as shown in B. 



the chloroplasts move about within the cell in order to be in the 

 most favorable place for manufacturing food. In very bright 

 light they place themselves where they will not be injured by it, 

 while in weaker light they come nearer to the surface of the leaf. 

 This is seen to especial advantage in the cells of moss leaves and 

 fern prothallia. In moderate light the chloroplasts gather along 

 the walls at right angles to the incident rays, but in intense light 



