THE METHOD OF MOVEMENT 183 



which in turn can be seen to move faster than the chloro- 

 plasts. In the highly speciaUzed sieve tubes, it is con- 

 ceivable that the strands of protoplasm, or certain regions 

 of them perhaps carrying sugars and other solutes, may be 

 moving at still greater speeds. 



Schumacher (1933) has demonstrated a movement of 

 fluorescein through the protoplasm of sieve tubes at rates 

 approximating 5 to 6 mm. a minute under optimum con- 

 ditions. He suggests, however, that the dye is not carried 

 by moving protoplasm (see Sec. 37), one reason being that 

 streaming, with the exception of that observed in plants 

 like Char a, is usually much slower than this. On the other 

 hand, it seems significant that this observed rate of move- 

 ment is within the range of possible streaming in specialized 

 cells. 



Although it seems generally agreed that there is no 

 movement of protoplasm through the fine plasmodesma, 

 it is possible that Kienitz-Gerloff (1891) was correct in 

 suggesting that there is such movement. Ewart (1903, 

 p. 30) claims that pressures necessary to cause flow through 

 such pores would be so great that it is safe to conclude 

 that no streaming movement in mass can take place 

 through them. If the flow is due merely to differences 

 in pressure at the two ends of the minute tube, his con- 

 clusion seems justified, but normal protoplasmic stream- 

 ing seems not to be caused by such pressure differences. 

 If normal streaming in fine protoplasmic threads is not 

 due to pressure differences at the two ends of the thread, 

 it is conceivable that flow through plasmodesma and 

 especially that through the coarse connections of sieve 

 pores may take place independently of ordinary pressure 

 differences. Much of the resistance to flow through 

 capillaries is, of course, due to friction, but when forcing 

 liquids through capillaries filled with air or other liquids, 

 a large part of the resistance may be due to surface tension. 

 The surface tension and electrical conditions in flowing 

 protoplasm may be such as to enhance flow and not resist 

 it. The probability (denied by some investigators) that 



