The nature of response to light in Amoeba proteus (Leidy)
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Amoeba proteus orients in directive illumination and is negative in strong light and probably positive in very weak light.
Negative orientation is due to inhibition in the formation of pseudopods on the more highly illuminated surface; and this is due to increase in the elastic strength of the plasmagel on this side.
If the luminous intensity is rapidly and greatly increased, all movement ceases, but if it is only slightly increased there is only a slight momentary retardation in the rate of streaming and this is confined to the tip of advancing pseudopods. Between these extremes there are innumerable types of responses depending upon the magnitude of increase in luminous intensity.
Cessation in streaming is due to gelation produced by the light, resulting in increase in the thickness and the elastic strength of the plasmagel sheet at the tip of advancing pseudopods.
The magnitude of gelation is correlated with the extent of increase in luminous intensity. This accounts in part for the relation between the nature of the response and the extent of increase in luminous intensity.
The smaller the increase in luminous intensity, the less the gelation and the less the retardation in rate of flow. This relation continues theoretically until the increase in intensity is reduced to zero.
There is no threshold and the “all or none law” does not apply.
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