CHAPTER II 



PHYSICS OF STREAMING 



SECTION 2. Mechanics and Mechanical Models. 



THE first of the physical problems connected with protoplasmic 

 streaming which require solution are : (i) the source or sources of energy, 

 (2) the character and mode of application of the force or forces inducing 

 movement, (3) the amount of work done in overcoming resistance. The 

 first two of these questions can only be answered in general terms, but 

 the third can readily be solved with approximate accuracy by indirect 

 experiment. It will tend to a clearer understanding of the problems 

 at issue if we first consider them in simplified form in relation to 

 a mechanical model. 



If a cylinder of water is placed vertically, and one side heated while 

 the other is cooled, the convection currents set up will assume a regular 

 direction around the long axis of the cell. This was first used as an 

 illustration by Dutrochet 1 , but it is an imperfect one, for the current 

 can never be mainly horizontal, as it may be in elongated living cells. 

 A better model is formed by a metal box, filled with cold water, and 

 having a number of obliquely inserted tubes closed at their outer ends 

 inserted in its walls (Fig. i). The outer ends of the tubes are strongly 

 heated, and the escaping steam imparts by intermittent impulses an 

 onward movement to the surrounding water before it is entirely condensed 

 again. By using tubes of fairly large bore, and inclining them slightly 

 downwards, they may be kept filled with water without any appreciable 

 backward flow being set up. When in action the outer layers of water 

 will be at rest, further inwards the velocity steadily increases to a maxi- 

 mum, then falling again to nil at the centre of the cell. The wall of 

 the box corresponds to the cell-wall, the outer stationary layers to the 

 ectoplasm, the inner rapidly moving layers to the endoplasm, while the 

 central portion, which is moved by friction against the layers outside it, 

 corresponds to the cell-sap, whose motion appears to be passively induced. 



If the box were closed, provided with a safety-valve and placed 

 in an erect position, the force of gravity would counterbalance on* the two 



1 Ann. sci. nat., 1838, ii. s^r., T. ix, p. 24. 



