Physical Aspects of Protoplasmic Streayning 207 



IV. MEASUREMENT OF THE MOTIVE FORCE 



By controlling the air-pressure in B, it is possible to oppose the 

 motive force in such a way as to hold the protoplasm at a standstill. 

 The counter-pressure, which is just sufficient to prevent the proto- 

 plasm from flowing either forward or backward, is a measure of 

 the absolute value of the motive force responsible for the streaming 

 of the protoplasm. This counter-pressure has been termed "balance- 

 pressure" (Kamiya, 1940) . 



The range of the absolute value of the balance-pressure is 

 usually within 20 cm. of water. But this value, as will be shown 

 later, varies from plasmodium to plasmodium of the same species, 

 and from rhythm to rhythm of the same plasmodium under the 

 same external conditions. The maximum value so far encountered 

 was 30 cm. of water. 



Since the motive force developed in the protoplasm does not 

 remain constant, the balance-pressure must be adjusted accordingly, 

 if the protoplasm is to be kept immobile. In order to do this, one 

 must constantly watch the direction tendency of the moving proto- 

 plasm at some definite part of the connecting strand. As soon 

 as the equilibrium between the natural motive force and the balance- 

 pressure is broken and, consequently, the tendency of the flowing 

 direction is recognizable, the counter-pressure is increased or de- 

 creased, as the case may be, so that the protoplasm under observa- 

 tion always stays at the same place. With a little practice and skill 

 this procedure enables one to restrict the movement of the proto- 

 plasm within a range of 50ii. So sensitive is the movement of the 

 protoplasm that the slightest deviation from the balance point will 

 induce movement in an 8 mm. strand.^ 



When the protoplasm is kept motionless at a definite point in 

 the connecting strand, there is no longer a mutual transference of 

 the protoplasm between the two parts of the plasmodium. However, 

 it must be noted, that suspension of flow of the protoplasm under 

 observation does not mean the cessation of movement elsewhere 

 in the plasmodium. Local displacement of the interior protoplasm, 



^This fact does not, however, necessarily mean that protoplasm has no 

 "yield value," since there is a possibility that displacement due to the slightest 

 shearing stress may be ascribable to an elastic deformation. The inherent 

 motive force which cannot be eliminated under normal conditions makes it 

 impossible to determine whether or not the slightest shearing stress is capable 

 of allowing protoplasm to undergo continuous deformation (flow), or whether 

 a certain amount of critical shearing stress is necessary to make protoplasm 

 flow. 



