INFLUENCE OF VISCOSITY OF PROTOPLASM AND CELL-SAP 17 



at all l . As the temperature rises and the velocity increases, the outer 

 layers of cell-sap begin to move with increasing speed, but the movement 

 never extends far inwards in large cells. The mechanical moment 

 exercised by rotating fluids in virtue of their viscosity has been investigated 

 by Mallock 2 , but in the case of a rotating cell, to determine the force 

 expended in unit time in moving the cell-sap, it would suffice to know 

 the average velocity of the latter, the mass of it moved, and its viscosity. 



An increase in the velocity of the endoplasm will not only increase 

 the bulk of rotating sap but will also tend to bring the outer layers of the 

 protoplast into motion. In Chora and Nitella the ectoplasm and the bulk 

 of the chloroplastids always remain at rest, but in Elodea, Vallisneria, &c., 

 the whole of the protoplasm and chloroplastids may appear to rotate. 

 The outermost layer must, however, for both physical and biological 

 reasons, remain at rest, and in fact examination with high powers always 

 reveals the existence of a peripheral non-moving layer * wetting ' and 

 adherent to the cell-wall. Hence as the velocity increases, the most 

 actively moving layer travels outwards, and as it decreases moves inwards. 

 Moreover, the velocity falls very rapidly towards the outer non-moving 

 layers, but much more gradually towards the cell-sap. The latter fact 

 points to a relatively high viscosity of the protoplasm as compared with 

 that of the cell-sap. 



It is obviously impossible to measure the viscosity of protoplasm 

 directly, but approximate measurements can be obtained by indirect 

 observations, and by comparison with allied substances. Thorpe and 

 Rodger 3 have obtained some very interesting results bearing upon the 

 influence of chemical constitution upon viscosity. Thus in homologous 

 series the viscosity increases as the molecular weight does, but it is also 

 influenced by constitution and complexity. There is, however, no absolute 

 relation between molecular weight and viscosity, while in the case of many 

 proteid substances, a relatively trifling chemical change (coagulation) may 

 enormously increase the viscosity. 



The osmotic concentration of the cell-sap is usually equal to from 

 i to 3 per cent. KNO 3 , and it usually contains less than 5 per cent., rarely 

 more than 10 per cent, of dissolved matter. The viscosity of a watery 

 solution is usually greater than that of pure water. Thus a 21-5 per cent, 

 solution of cane sugar has twice (17 = 0-0202), a 30 per cent, three times 

 (17 = 00304), and a 40 per cent, six times (77 = 0-0607) that of water at 

 20 C. (17 = 0-01009). Taking water as unity, the viscosity of normal 



1 The currents in the cell-sap may be rendered more clearly visible by causing the cells to absorb 

 aniline dyes, or producing granular precipitates in the cell-sap by treatment with dilute solutions of 

 ammonium carbonate, or of caflfein. 



Phil. Trans., 1896, vol. CLXXXVH, p. 41. 



3 Phil. Trans., 1894, Bakerian Lecture. 



