96 AMEBOID MOVEMENT 



distinction seems to be of little significance, for the same cell 

 may at different times show both types of streaming. When there 

 is a single vacuole only in the cell, it occupies the center of the 

 cell, and the endoplasm then rotates between it and the ecto- 

 plasm. Whenever there are strands of endoplasm flowing across 

 the vacuole, the peripheral streaming is no longer rotational but 

 it is then called circulatory. By external stimulation of the cell, 

 Ewart ('03) was able to change circulatory streaming into rota- 

 tional ; that is, the numerous small streams traversing the cell sap 

 in many directions were caused to retract into a single stream 

 around the periphery of the cell. This change brought about a 

 heightened velocity in streaming, showing that the small strands 

 traversing the cell sap meet with some resistance. There is no 

 essential difference between streaming in plant cells, whether rota- 

 tional or circulatory, from the rotational streaming so commonly 

 found in protozoa. 



Ewart has also observed that in the streaming of the endoplasm, 

 there is a variation of velocity of streaming in different parts of 

 the stream (Figure 31). The middle of the stream moves fastest 



o b c d 



Figure 31. Diagram of a section of a Chara cell showing rows of 

 emulsion globules in the endoplasm, after Ewart. a, cell wall, b, ectoplasm. 

 c, endoplasm, d, cell sap. The arrows at the top of the figure indicate 

 by their lengths, the amount of movement of the endoplasm and cell 

 sap in streaming. 



while the layer near the ectoplasm moves very slowly and the layer 

 in contact with the ectoplasm moves hardly at all. But the endo- 

 plasm in contact with the central vacuole moves only a little more 



