Some Physical Properties of Protoplasyn 255 



The mechanism of protoplasmic flow has long been a subject of 

 speculation. Change in surface tension, the sine qua non of so many 

 cellular activities, was once regarded as being responsible for the 

 streaming of protoplasm. There was also the suggestion that the one- 

 way shuttle type of flow in the filaments of coenocytes where move- 

 ment is first in one direction and then in the other, may be due to 

 hydration at one end and dehydration at the other end; but the 

 protoplasm flows equally well in both directions, even when fully 

 submerged. 



With the introduction of colloidal chemistry into physiology it 

 became apparent that many cellular activities had their counterpart 

 in colloidal systems. There thus arose the suggestion that proto- 

 plasmic streaming is a cataphoretic migration of particles or the 

 electroendosmotic flow of an aqueous medium; if either, it is the 

 latter, for in streaming protoplasm the entire mass of material moves 

 and not just the suspended particles. In spite of some attempts to 

 prove that streaming protoplasm is associated with electric poten- 

 tials, there has been no convincing evidence that the potentials 

 measured are real in the sense of innate to the protoplasm, and if 

 real, are the cause rather than the result of the streaming. 



One difficulty with most theories advanced in explanation of 

 protoplasmic movements is the lack of any explanation of a reversal 

 in the mechanism. The protoplasm of slime molds flows first in one 

 direction and then in the other. Reversal occurs normally every 45 

 to 50 seconds, the time of outward flow occupying about five seconds 

 longer than that of the inward flow, which accounts for the advance- 

 ment of the Plasmodium. Were a difference in potential responsible 

 there would have to be a change in polarity every 45 seconds, and 

 there is no evidence that such a change occurs, except the reversal 

 in flow. 



If time-lapse moving pictures of the streaming protoplasm of the 

 myxomycete, Physarum polycephalum, are made, a remarkable 

 pulsation of the plasmodium is revealed. Pictures taken every five 

 seconds and shown at the usual rate of sixteen a second, which 

 means a speeding up of eighty times, show the plasmodium under- 

 going rhythmic contractility and relaxation in perfect synchronism 

 with the protoplasmic flow (Fig. 3) . The rhythmic contractions and 

 expansions when thus optically accelerated resemble the pulsations 

 of a heart. At each contraction and relaxation the direction of proto- 

 plasmic flow reverses, and with the outward flow the plasmodium 

 advances slightly. 



