INFLUENCE OF GRAVITY UPON ROTATION 23 



frequent temporary formation of subsidiary obliquely directed ' currents/ 

 and occasionally a change in the plane of movement. The presence of 

 only a small amount of readily coagulable protcid will suffice to cause 

 a stoppage of streaming at its coagulation point, just as a small amount 

 (0-2 per cent.) of fibrinogen causes the clotting of blood when it turns into 

 solid fibrin. 



Cells which have been exposed to 50 .-5.5 C. for a short time 

 may recover, and show fairly active rotation at normal temperatures. 

 Frequently, however, the original rapidity is not regained for some time, 

 and the activity of streaming is in some cases permanently depressed. 

 The plasma of such cells may show a distinct tendency to ball together, 

 and the streaming layers may contain irregular transparent masses often 

 with imbedded chloroplastids. These masses are probably formed by 

 partially coagulated proteids and may subsequently disappear, apparently 

 being broken up and reabsorbed. 



SECTION 9. Influence of Gravity on Eolation. 



To drive the same bulk of liquid upwards through a capillary in the 

 same time as when it is driven downwards requires a greater pressure, or 

 takes a longer time if the pressure is constant. The difference of pressure 

 is proportional to the density of the liquid, the relative velocities of flow, 

 and the force of gravity. It does not follow that a corresponding difference 

 will be observed between the ascending and descending streams in a cell 

 placed with its long axis vertical, even in the absence of any physiological 

 response to gravity. For in a closed system of this kind, the action of 

 gravity counterbalances on the two sides. Thus in a series of observations 

 made upon small more or less cubical cells of Elodea and Vallisneria no 

 constant difference of velocity could be observed between the ascending 

 and descending streams. Frequently the endoplasm became thicker at 

 certain points, and these thickenings circulate several times around the cell 

 before thinning out again. If they are very marked, irregular variations 

 of velocity ensue. In the case of large elongated cells, gravity seemed 

 apparently to exercise a distinct influence upon the velocity of streaming. 

 The action was immediate and was not preceded by any perceptible latent 

 period, or by any after effect. Any stimulating action, it should be 

 noticed, would necessarily be slightly greater upon the ascending than 

 upon the descending stream. The observations were made upon large 

 elongated leaf-cells of Elodea and Vallisneria, upon end cells of Chara, and 

 upon internodal cells of Nitella, in which the direction of streaming was 

 more or less closely parallel to the long axis of the cell. 



The velocity of streaming can only be told by observing the time 

 taken by a floating particle to cross a measured space on a micrometer 



