36 PHYSICS OF STREAMING 



It is easy to prove that streaming is not directly dependent upon 

 supplies of radiant energy (thermal, photical, electrical) from without, for 

 its initiation and maintenance 1 , although indirectly it is dependent not only 

 upon the maintenance of a certain temperature, but also, in the case of 

 autotrophic plants, upon a supply of radiant energy in the form of light. 

 The intensity of the illumination also apparently exercises a direct influence 

 upon streaming, strong light retarding, and weak light often causing, 

 a slight acceleration. The latter is most obvious in chlorophyllous cells, 

 especially when the motion has become somewhat retarded in darkness. 

 It may be due either to an increased supply of oxygen from the assimila- 

 tion of internal or external supplies of carbon dioxide, or it may be owing 

 to the slight rise of temperature which the absorption of light always causes, 

 even when the dark heat rays are removed. Hence the acceleration is 

 especially noticeable when preparations kept in darkness and at a low 

 temperature for some time are illuminated. 



A temperature lying within certain limits forms one of the essential 

 conditions for streaming, the response being almost immediate. The 

 velocity increases as the temperature rises, until the protoplasmic mechanism 

 is injuriously affected. The increased velocity is in all cases partly due to 

 the decreased viscosity which accompanies a rise of temperature, and in 

 some cases may have practically no other origin. If the energy of stream- 

 ing was directly derived from the absorption of heat from without, then, 

 since the amount required is extremely small, the lowering of temperature 

 by this cause would be quite imperceptible under ordinary circumstances. 

 Evidence has, however, already been given against this possibility. More- 

 over, the production of heat by moderately active katabolism would much 

 more than suffice to counterbalance any that might be consumed in this 

 manner. 



Both light and heat probably affect the protoplasmic mechanism as 

 a whole, and hence influence cell-division, nutrition, growth, and katabolism, 

 as well as streaming. As in the latter case, a part of the increased activity 

 of growth consequent upon a moderate rise of temperature may be due to 

 the decreased viscosity and hence increased motility of the protoplasm 2 . 

 There can be no doubt, however, that the effect is largely an indirect one, 

 in so far as it is due to the accelerating influence of the raised temperature 

 upon metabolism in general. 



Mechanical injuries, and indeed intense localized applications of energy 

 in any form, often induce streaming in previously inactive cells. At the 



1 For method and apparatus used, cf. Ewart, Journ. Linn. Soc., 1897, Vol. xxxm, p. 123 (proof 

 that the evolution of oxygen from certain coloured organisms is independent of external radiation). 

 3 Diffusion and ferment-action are also accelerated. 



