Page:Proceedings of the Royal Society of London Vol 69.djvu/489

Rh Of the different constituents of the cell, cellulose, albumin, and chlorophyll are paramagnetic ; starch, sugar, oil, water, and probably myosin also, are diamagnetic. Plant-cells usually, though not always, place their long axes parallel to the lines of force in a magnetic field.

The strongest magnetic field used exercised little or no direct effect on streaming, although a pronounced secondary effect is produced on prolonged exposure as the result of inductive action.

The connection between certain forms of streaming movement, such as occur in a few fungi, and metabolism is a wholly indirect one, but this can hardly be a general rule. Indirect relationships exist between streaming, growth, and assimilation, but no direct ones. Similarly the nucleus exercises no direct but a pronounced indirect influence on streaming.

An organised arrangement of the emulsionised protoplasmic particles is probably an essential condition for regular continuous streaming. A great variety of agencies when suddenly applied seem to disturb this arrangement momentarily, and hence produce a temporary cessation of streaming. This shock-effect results from sudden changes of concen- tration, sudden falls or rises of temperature, momentary electrical excitation, and the sudden application of various poisons.

The minimal, optimal, and maximal temperatures for streaming vary according to the plant or cell examined, and also depend upon (1) the age or condition, (2) the external medium, (3) the duration of the exposure, (4) the supply of oxygen, (5) the rapidity with which the temperature is raised or lowered.

At temperatures above 3(T C. the velocity immediately assumed is, in the absence of a shock-effect, always greater than that shown a few hours or a few minutes afterwards. Between 10 ' and 30 C. the permanent velocity is almost immediately assumed. Below 10 C. the acceleration due to a rise of temperature frequently does not become fully manifest until after a certain lapse of time.

In the case of facultative anaerobes, the response to changes of tem- perature is less pronounced in the absence of oxygen than in its presence. With short exposures the optimum and maximum points are raised, but with prolonged ones the maximum temperature is lowered by the absence of oxygen.

Strong light retards streaming, while weak light may indirectly accelerate it in chlorophyllous cells. It is still doubtful whether streaming is affected by directly impinging electro-magnetic wave- vibrations other than those of light.

Mechanical disturbances may act as inhibitory stimuli, and may be propagated internally in the form of pressure-waves.

Food-materials exercise both a direct and an indirect effect upon streaming. Acids, alkalies, and metallic poisons all retard streaming, and may cause a temporary shock-stoppage when suddenly applied.